The answer to this will depend on the variant of English which you are using. For British English speakers, being 'in school' does not necessarily refer to the status of being a student. I would tend to use 'at school' for that meaning. For example, you might hear a British parent say "I've got three kids: the eldest is at university and the two younger ones are still at school. It's really hard having them all at home during the lockdown!" This describes their stage of education, not their physical location. We would certainly understand the American use of 'in school' in this context, and - if Miriam's comment is anything to go by - it seems that many younger British people have even adopted this convention. Conversely, 'in school' can be used to mean "present on the school premises" in British English. For example, if a telephone call comes through for the school's head teacher, the head's assistant might answer "I'm sorry. She isn't in school today. Can I take a message?". That sounds perfectly normal to me. You could also use 'at' with the same meaning. I think this is a situation where American English conventions are stricter than British English ones.

No you should say “I’m at school.” Using “in school” means you’re a student... -Do you have a job? -No, I’m still in the school!

I'm at school right now. If you say I'm in school, it sounds like you're still a student.

Thanks!

A native speaker would say 'I'm at school'. People will know what you mean if you say 'don't call me now, I'm in school'. But normally if somebody simply says 'I'm in school', that means you are probably 5 - 18 years old and still in the education system as a student. 'At' clarifies that you are just physically at the school building, exactly like if you were 'at the doctor's' or 'at the library'.