Because the "in schools" changes the calculus massively. You see a similar reaction to banning phones and personal computers while in school.

Kids can still access and use AI outside of school meaning the enforcement is entirely on the school and doesn't require massively a privacy invasion for adults by way of age verification. Schools already put children under (from an adult perspective at least) a pretty intense rules regime so this doesn't represent any kind of change—indeed it's probably better framed as a lack of a change from the status quo.