I suppose technically it´s not good grammar but it´s often used - ´when I´m in school I´m not allowed to wear earrings´ - sounds okay to me - probably more American English than British English - but then those bed Amerikans are taking over the world!!!

I always say "I´m in school" and not "I´m at school". Teşekkürler to everyone who has helped!

For the Turks (and others) who want to learn English:

I´m in school either means that you are in the school building (but then it would be better to actually say "I´m in the school") or it means that you are a student. It is not something you are doing at the moment, but something that is true every day. For example, "Since I´m in school, I´m not allowed to wear earrings anymore"

I´m at school means what it means It is saying something about an exact moment. For example, "When I´m at school, I´m not allowed to wear earrings."