Qual è la differenza tra I'm at school e I'm in school e I'm in the school ?Sentiti libero di rispondere con un esempio.

Yes, or you might want to use "I'm at school" if the name is being specified. For example, "I'm at Harvard"
"I'm in school" means that you're inside the building and also what you said.
"I'm in the school" isn't used at all. The only example I can think of is when you're asked where you are at school when there are more possible answers such as the yard or the playground so you just specify where you are.
"Where, in the bedroom, is your jacket?" "In THE wardrobe." As you can see, there are more options, but in this example, you're basically pointing at the wardrobe from all other places in the bedroom.
Hopefully this will help you