Hi I usually use "at" to inform about a place I am, but can I say "I will be in the school at 8:15 a.m." , thus I want to emphasise that at the time I will be inside a school, indoors. not running to, not going upstairs or opening doors, I will be in the school ready to study?
You are correct that at 8:15 am means that you will be doing whatever it is that is described. However, "in the school" does not at all imply that you will be ready to study. You would need to say:
I will be in the school ready to study at 8:15am.
other prepositions work as well for this:
I will be at the school ready to study by 8:15am.
The latter implies you might be early. So at implies more precision, but not readiness. Technically the first sentence could say "at the school" and be correct, but it would just sound weird to use "at" twice in once sentence.
You can, but I don't think it has much emphatic effect. It suggests that you will be in the building, but it doesn't suggest that you will be ready to learn. In fact, it only emphasises physical location, so you could be "opening doors or running upstairs" but still be "in" school.
Students must be in school by 8:15 am (provided you are inside the building you are okay)
There is an idiom: "at your desk"
Students must be at their desks by 8:15 am (you must be at school and sitting down in the classroom. You must not be going upstairs etc)