Hi members of P.P.R.,
Schools can be notorious for paranormal activity. Without our doing an investigation, I couldn't say exactly what is happening at your school, but it doesn't sound like anything to be concerned about. Consider it to be former caretakers and teachers that are still doing their best to watch over the students.
SJGR would need to be invited by the school in order to conduct an investigation, but you may want to talk to some of your teachers or principle to see if they would like to have SJGR come to your school for a free lecture. You would all learn more about the paranormal and have an opportunity to speak directly with SJGR investigators.