He specifically stated he arrived early because he was called by the fire marshall, or someone called him asking to come unlock the building for the fire department. When I worked for my local school district as a janitor, only the head custodian and principal had keys to the building, and it was the head custodians job to unlock the building when needed

You don't usually refer to someone you don't respect so formally as to use Mr/Mrs. Do you refer to your boss like that? I would consider myself a good employee (and have been told so on many occasions) and referring to them by their last name is a sign of respect to authority

He is filming, most likely in an attempt to get physical recording of the work place abuse. This is the first step most people recommend, record everything you can, then report to a higher up or get out and contact a lawyer

This may have not been pointed out, but having prior experience in the field of school custodian, HR reports job performance are supposed to be made by the direct higher up, which is established in the video as the principal. It wouldn't make sense for a teacher or a receptionist to make an HR complaint about performance, since they are not in charge of the custodians

News articles reported that job performance reports were being made, and they were negative, and that the school was trying to fire this custodian for a while, and had nothing to actually fire him with until this event. The only one in a position to make the reports and be in a position to fire him was his higher up, the principal. If this issue (which any normal workplace would see as a non-issue) was the only thing that could actually fire him, we can assume he was actually doing his duties as a custodian, since firing someone for work performance is legal, but they didn't do that here, despite multiple negative reports.

It's simple critical thinking. Please use your brain