1. If we don't go to school, we go to academies.

Question

2. If we don't go to school, we go to an academy.
3. If we don't go to school, we go to academy.
4. If I don't go to school, I go to academies.
5. If I don't go to school, I go to an academy.
6. If I don't go to school, I go to academy.
[Which sentences are ungrammatical? Would you check articles and plural forms at the end of each sentence?]

Answer 1

1 is fine
2 is fine
3 - no. If there is only one academy, we need to say "the academy". If there are more than one, we need to say "an academy." Only if the academy's proper name is Academy may we omit the article.
4 - no. Do you (singular) go to several academies, more than one?
5 - is fine
6 - see #3 comment above.