To summarise up front, he used the word imbécile which is more or less like "idiot", but he used it as an element of an idiomatic figurative phrase which means "you can play-act someone who doesn't care or doesn't know better" or "in a remarkably (and in this case inappropriately) juvenile way" -- not saying that "you 'are' an idiot", saying "you can 'play' an idiot".
In my opinion, this is different for three reasons:
- It's literally different -- using the verb faire (Tu peux faire l'imbécile) instead of être (Tu peux être imbécile)
- It means something different -- e.g. to 'be' an idiot is permanent and not a matter of choice, to 'play-act' was temporary (maybe even comic in the right circumstance)
- In any language I've been told that it's better if a teacher criticises behaviour than criticises the person -- better to say for example "that was naughty behaviour (which you needn't perpetuate)", rather than "you are a naughty boy". And it's better to make constructive criticism.
Faire l'imbécile is a specific phrase:
s'amuser de façon espiègle, folâtrer
It's literally to "make (like) an idiot" or to act out -- he's not saying that the boy is an idiot -- he is accusing him of fooling around, perhaps of mockery.
Given the Chant des Partisans I take it that the occasion was a World War Two memorial, to honour and remember those who resisted and were killed -- and that M. Macron was there to represent the French State's honouring the fallen -- not a political occasion.
I understand (having attended similar ceremonies) why he might be prickly about it. I don't think he was especially prickly, more pedagogic. The boy's calling him "Manu" stuck out like sore thumb, and it's so informal that it's, in my opinion, obviously intended to be disrespectful, or at best inviting M. Macron to share the joke.1
Here is a transcript of the French:
"Tu es là dans une cérémonie officielle. Tu te comportes comme il faut", poursuit alors Emmanuel Macron. "Tu peux faire l'imbécile. Mais aujourd'hui c'est la 'Marseillaise', et le 'Chant des Partisans'. Tu m'appelles 'Monsieur le président de la République' ou 'Monsieur'".
You are (there) in an official ceremony. You behave yourself properly (literally, 'as it requires'). You can play the fool. But today, it's the Marseillaise, and the Chant des Partisans. You (must) call me "Mister the President of the Republic", or "Mister (i.e. Sir)".
It's also worth mentioning that, in my opinion, the French have an old-fashioned notion of politeness and formality, beyond what I think is normal in the UK or the USA (see e.g. the LA Times' article on when to use Tu as opposed to Vous for example), and which is taught in schools and by adults.
I'm not sure I'd call it "a public dressing down" even -- more like a correction. I get the impression from the video that M. Macron went on, went back, kept talking, in order to try to help, to find some good and agreeable advice to offer: "do things in the right order, first get your diploma."
Some people (in comments) are wondering why he talked so long.
It seemed to me that, having started their conversation on the wrong foot, "Manu" was trying to find something conciliatory, some common ground -- i.e talking about school, knowing this is exam-time, encouraging him to do well -- with body language like patting him on the arm, twice, as if trying for a rapprochement -- and asking him if agreed (d'accord?), and saying "I'm counting on you (to be exemplary in future)" -- he even said "je compte sur vous" (not "sur toi") by the time he got around to saying that, i.e. addressing him respectfully as an adult/citizen (or as representing a plural).
The Tweet says,
Le respect, c’est le minimum dans la République – surtout un 18 juin, surtout en présence des compagnons de la Libération. Mais cela n’empêche pas d’avoir une conversation détendue – regardez jusqu’au bout.
Respect is the minimum in the Republic - especially on June 18, especially in the presence of the companions of the liberation. But that doesn't prevent having a relaxed (informal, friendly) conversation -- watch (this video) to the end.
Note the definition of respect
- Sentiment de considération envers quelqu'un, et qui porte à le traiter avec des égards particuliers ; manifestations de ces égards : Manquer de respect à quelqu'un.
- Sentiment de vénération envers ce qui est considéré comme sacré : Le respect des morts.
- Considération que l'on a pour certaines choses : Le respect de la parole donnée.
- Feeling of consideration towards someone, which sustains treating them with particular regards ; or the manifestation of these regards : (example of the word used in a phrase) to lack respect toward someone
- Feeling of veneration towards that which is considered sacred: (example of the word used in a phrase) respect for the dead
- Consideration that one has for certain things: (example of the word used in a phrase) respect for the given word (e.g. for keeping a promise)
I think that M. Macron is (whether or not you agree) hereby implicitly disclaiming having been disrespectful himself.
Also there's a related phrase in French, faire le singe -- literally "make the monkey", figuratively "act like a monkey (or a clown)" or "to clown around" -- because it's a standard figurative phrase, you'd be missing some of the nuance if you only translated each word literally.
It might be worth mentioning the place, too, which according to the OP was Mont Valérien:
Du lieu de l'Histoire au premier des Hauts lieux de la mémoire nationale :
Lieu de culte médiéval devenu forteresse militaire au cours de XIXème siècle, le Mont-Valérien a été le principal lieu d’exécution de résistants et d’otages en France par l’armée allemande pendant la Seconde Guerre mondiale.
La multiplicité des parcours des 1008 fusillés, nous permet aujourd’hui d’en décrire la diversité. Après la guerre, le site est choisi pour honorer la mémoire des morts pour la France de 1939 à 1945, et, le 18 juin 1960, le général de Gaulle y inaugure le Mémorial de la France combattante.
Ces hommes, assassinés parce qu’ils étaient résistants, otages, Juifs ou communistes sont autant de rappels à notre histoire qui firent naturellement de ce site le premier des Hauts lieux de la mémoire nationale du ministère de la Défense, aujourd'hui géré par l'Office National des Anciens Combattants et Victimes de Guerre.
From a place in History to the first of the High (important, altar-like) places in the national memory:
A medieval place of worship, become military fortress during the 19th century, the Mont Valérien was the principal place of execution of the members of the resistance and hostages in France by the German army during the second world war.
The multiplicity of the itineraries of the 1008 people shot, allows us today to describe their diversity. After the war, the site is chosen to honour the memory of those who died for France from 1939 through 1945, and, on June 18 1960, General de Gaulle inaugurated there the Memorial of France at war.
These men, assassinated because they were resistance, hostages, Jews, or communists, were as such (or were so many) reminders of our history who consequently made of this site the primary of the High places of the national memory of the ministry of Defence, today managed by the National Office of the Veterans and Victims of War.
Going back to the quoted definition, i.e.
s'amuser de façon espiègle, folâtrer
If I paraphrase these definitions of espiègle and folâtrer, then (together with my own understanding of the phrase), the definition says, more or less, "to amuse oneself" ... comic; transgressive but not mean, almost innocent; doesn't know better; unworried -- the implication is that the boy ought to show more respect than to act that way here -- it may also imply that we are not amused.
That's close to how Europe1's reporting after the fact characterizes the boy's greeting:
Alors que le président de la République saluait plusieurs jeunes attroupés derrière des barrières, l'un d'entre eux, après avoir entonné les premiers mots de l'Internationale, s'est amusé à apostropher l'hôte de l'Élysée d'un "ça va Manu" ? Mais le jeune homme a été immédiatement repris par Emmanuel Macron. "Non, non", a-t-il rétorqué, alors que le jeune garçon, semblant vite regretter cette familiarité, s'excusait : 'désolé monsieur le président".
As the president of the Republic was greeting several youths who were trooped behind barriers, one of them, after sounding the first words of the Internationale, *amused himself by abbreviating2 the host of the Élysée by a "ça va Manu?" But the young man was immediately taken up by Emmanuel Macron, "No, no", he replied, while the young boy, seeming to quickly regret this familiarity, was excusing himself: "sorry (or 'abject apologies') Mister President."
One more thing, he wasn't just criticising the boy's behaviour -- he was criticising his French, which I think of as normal and as natural as breathing, it's how people (children) learn French. There are varieties or registers of French, and you're taught to use the right register on the right occasion.
Where I'm living it's normal to say "Bonjour Monsieur !", "Bonjour Madame !", "Bonjour !" to everyone you pass on the street. I once absent-mindedly said "Bonjour Monsieur" to an approaching child, and the enormity of what I'd said made him catch his breath and correct me: "Je ne suis pas un 'Monsieur' ! Je suis en enfant !"
1 A comment below tells me that when the boy says, "ça va Manu?", that "Manu" is an abbreviation of "Emmanuel (Macron)". So it's a bit analogous to greeting the Queen of England or the Prince of Wales as, "Hey Lizzie!", or, "How's it going, Chuckles?"
2 The French verb apostropher dosn't even have an equivalent in English that I can think of. Its definition is, "Adresser brusquement la parole à (qqn), sans politesse", i.e. "address speech to someone brusquely, without politeness."