I urgently need some advice!

And is Denis the same age as him? If not, let him just forget about it. If yes, suggest that they "explain" to Pasha together what exactly he is doing wrong. Also, get Sanya involved, since it all started with him. Then it will be that Pasha and his friends are not against Denis alone, but against a group of friends. This will be the best solution, in my opinion, because at that age, people don't know how to "just forget about it," and besides, they are guys, and they need to defend their right to their place in the sun.