Wht not the?
Because ann an is essentially one word meaning in. It just happens to have a part which is the same as the an which is often used as a word for "the".
In the school would be anns an sgoil, with s added to the end of ann.
How would you say "at school" meaning "in class"? Would it also be "anns an sgoil"?
no. That would be "in the school".
In the class would be "ann an clas".
Well, in the class would be anns an clas.
If you're wanting at school I'm tempted to think aig is the right preposition. Tha mi aig sgoil - I am at school.
I thought "ann an sgoil" would equate to "at school" - but it seems like this phrase signifies physically being in a school building, rather than being there as a pupil. Hmmm.
Why is "at school" incorrect please? Surely that's what we would say in English?
You don't know the context for this, so stick to the grammatical meaning is my advice, especially when they're trying to teach prepositions. ann an = in, aig = at