English- 'I'm in school' or 'I'm at school'?!

Either works.

'I'm in school' indicates that you are somewhere (likely the school building, but maybe on like a field trip or something) with a teacher FOR school. It can also mean that a break hasn't come yet, like Winter or Summer break.

'I'm at school' indicates that you are physically at your school building, again probably for school (like learning). If you say 'I'm at THE school' it would mean that you are at the school building, although not necessarily FOR school.

So sorry... I'm awful at explaining.

Me...

Yes, you can. However, 'at' describes the exact moment about which you are speaking, i.e. "I'm at school" implies "I'm at school right now".
"I'm in school" can also be used in this context but it could also be used to answer a general question. Example:
- "Do you work or do you study?"
- "I study; I'm still in school."

Native speaker

"I'm IN school" means that you are currently enrolled in a school somewhere, even if classes are not currently in session, like over summer break
"I'm AT school" means that you are curently physically located within the school building or on school grounds.