This answer talks about two Ukrainian photojournalists, both called Sergei: Sergei Supinsky and Sergei Dolzhenko.
Both of them photographed the training event in Ukraine.
Supinsky's photos are available from Getty images where they are described as:
TOPSHOT - A military instructor teaches civilians holding wooden replicas of Kalashnikov rifles, as they take part in a training session at an abandoned factory in the Ukrainian capital of Kyiv on January 30, 2022. - As fears grow of a potential invasion by Russian troops massed on Ukraine's border, within the framework of the training there were classes on tactics, paramedics, training on the obstacle course. The training is conducted by instructors with combat experience, members of the public initiative "Total Resistance".
Dolzhenko's photos are available from the European Pressphoto Agency, where they are described as:
Open military exercise for civilians in Kiev
Ukrainians attend an open military training for civilians on a training range in Kiev, Ukraine, 30 January 2022. [...]
Neither explicitly mentions schoolchildren, although some of the subjects are clearly school-aged.
As tensions between Russia and Ukraine are high, we might expect to see propaganda from each side. It would be well to consider the propaganda and military recruiting value of such a training day, compared to how much more effective it made the participants in any combat situations.
(Hat tip to pinegulf's answer, which introduced me to Supinsky. I was going to make a minor edit, to add a link to Dolzhenko's photos, got confused by the surname mismatch, and decided it should be a separate answer.)