It should be easy to prove that it is edited but considering this is a school we are talking about, I doubt they would actually put effort into determining the authenticity.
Edit:
Yeah, just look at the picture. It is clear it's just an amateur edit. School teacher is actually not as dumb as often potrayed in fictional stories. Experienced teacher had many opportunities to deal with problematic students so whether they liked it or not, many of them are sensitive to these kind of things. The problem is now whether the teacher give a fuck or not, or whether the school will take action or not.