1. une école is for students aged and is like an american school.

un collège is for students aged and is similar to a school.

un lycée is like an american school.

2. preposition à means to, and in.
à + le changes to:
à + la stays and à + l’ stays .
à + les changes to:

1;
Une école is for students aged 6-11 (I've heard some people group it 2-11) and is like an American elementary school. Un collège is for students aged 11-15 and is similar to a middle school.
Un lycée is like an American high school.
2;
à is to, in, at.
à + le is au.
à + la = à + la    à + l' = à + l'
à + les = aux
(These rules are similar to de.)

They don’t cuz it’s dumb