The problem / challenge there is that school isn't very stimulating, or, not for everyone - school, especially elementary and secondary school, are based more on discipline and forcing people to learn a basic curriculum, whether they like it or not. If they get distracted by their phones or whatever they'll run behind, score low on their tests, end up in a lower secondary / tertiary education, aaaand have to settle for an okay job.

Kids are very easily distracted, and the current school systems are designed to deal with those by removing distractions and forcing a bit of discipline.

The alternative would be more personally optimized education, but that's a lot more expensive and harder to scale - and teachers are already an underpaid and overworked demographic.

Besides, kids that age use their phone more for social contacts and distractions than learning stuff.