My school is supposivly haunted my friends and I have been investingating it we see faces papers move from are stand lights come on and off and doors open by themselves big bangs occur, as well as shuffling noises, pictures are taking and people see the ghost how can I find out if this is a spirit?

Do you have the various meters to detect the energy fields. Do you have video equipment to set up and leave to film things happening.

A person does not need to have died for a spirit to reside somewhere.

The best way to know if something is a spirit or not is to rule out the natural explanations for the occurrence in question... not always an easy thing to do.
I might add that if you are investigating your school, make sure some teachers or school officials know about it before you go snooping around in places that you shouldn't.