It would help if you copied and pasted your exact answer so we could help you see if there was some other error or typo that tripped the correction algorithm. Just because it provides "...in school" for the corrected answer does not necessarily mean the problem with with "at" vs "in".
It depends what kind of schools!
There are differences between schola and ludus
= Roman primary school.
= Roman middle school, from age 12.
Study: Latin, Greek, Literature, and grammar of course.
schola Platōnica (for instance with Platon)
= Followers of a philosophe.
Now I understand why "ludus" also mean "game" etymologically, because it's like a kindergarten (even if they are older, it's the idea behind).
Ludus is for small children, and schola for adults and teenagers.
The next time that happens, take a screen shot and file a bug report.
Wouldn't "at" school be better? Although "in" may not be totally unacceptable, if "at" is better, one should not prefer "in" because in Latin is also "in". That would be a false friend, o a partially false...sometime English "in" is the best translation for Latin "in", but other times, like in this case, in seems to be a deformation because the words are similar or the same, but not suitable.
A translation should not be literal, word by word, nor keepinh a word if it is same. A good translation translate the meaning to proper English, and not to a stranger standard that's accidentally similar to Latin. It should not sound or look similar, but mean the same.