Are you missing a negative in this sentence, or are you going Chin on us?
They got back into formation, waited, and then attacked on the last play of the game. That by itself is evidence of premeditation. The fact that it was the last play doesn't help their case, since it would be harder to argue it was in the heat of the moment if they waited until the consequences of getting kicked out were lowest.
I have seen parents act like jerks, and some of the worst behavior is at men's rec games. There actually was a ref killed by a sucker punch at a men's soccer game in Michigan a few years ago. I don't think I have ever seen or heard about a planned attack on a ref on the field. That is the worst part of this in my opinion, and why I am suspicious about the coach's involvement or at least pre-knowledge of the attack.