The fact to remember is, this is school rugby. Yes, it is a fair tackle, but really, is it necessary to risk serious injury to the guy you are tackling by going into the tackle like that. Play hard, but play fair. What if you were the much smaller guy on the receiving side of the tackle. The force was totally unnecessary.
I can understand why the ref called it a penalty at this level... it's clearly not, just a monstrous tackle, but in real time if you weren't focusing on the contact.. that'd be hard to tell, meanwhile ol mate is in a coma now