This is a (too long) comment about the Swedish study, in addition to particularly @igelkott's link about balancing the risks of death by head trauma and death by cancer due to the unnecessary CT.
I'm wondering how much can be skeptically concluded from the The Swedish study. They list as participants
Participants 3094 men who had received radiation for cutaneous haemangioma before age 18 months during 1930-59.
In other words, the studies population are boys who got radiotherapy because of tumours on the skin of the head or somewhere else. I assume (though I did not find that clearly stated in the text!) that the controls are boys that got radiation therapy for this type of tumour somewhere else than the head and neck region.
IMHO the more interesting control group would have been boys who had the hemangiomas on head and neck but did not get radiation therapy (stratified for severity of the tumour / the radiation dose they "should" have gotten). Or at least, one would need to establish that high school attendance of people with untreated infantile hemangiomas was the same compared to boys without hemangiomas (probably not, if "only" because of the psychosocial difficulties in school).
Now according to this webpage these hemangioma are benign tumours, 60% of them appear on head and neck. But a particular form of these hemangioma appears within the PHACE syndrome, which also includes brain malformations and other problems. I have no information about the frequency of this, but it would have been good if the Swedish study had explained if any/how many such patients were in their patient population.
The other point that needs to be kept in mind is that according to Benner 2001 the cancer risk decreases with the age of the child when the CT takes place. The Swedish boys were less than 1 1/2 years old when they got radiotherapy. Now cancer isn't the same as not attending high school, but this tells something about how sensitive the brain reacts to radiation.