Pretty bizarre that someone would record this so...quickly, while it’s happening...without running for their life???
Maybe I’m too cynical?
As a dad of a 15 year old, you are too cynical. Kids consider their phones an extension of themselves and they have them in their hands at school and everywhere else. This kid appeared to be recording his/her papers or perhaps the teachers lecture when the shots rang out and quickly stopped filming. My instant analysis: There’s nothing sinister there.