My friend is being threatened bullied?

Okay, so my friend offended my clmate by telling the teacher that the clmate good friend had been playing with the bunson burner. So now my friend is getting girly bullied and verbally, he is very small in size only like 1.40m and the bully is 1.65m, till now, hes been arguing with him, flicking spiders at him and not ping him the paper work sheet. and i overheard his conversation trying to instill fear into him by saying, "When I was in school, i used to get in fights and i don't care if i get expelled" I actually felt kind of irritated when he said that. i am scared that he will beat my friend when he goes back home. it seems that my friend hasn't told anyone yet. thought i noticed it and quite a number of cl mates, but though they are all too afraid to face gangster things. what should i do? i really dont think i should just stand here and do nothing, i feel its going to explode into something large like putting something in my friends bag and accusing him of doing it, i just know he's going to do it! and when he does, no one will stand up for my friend. oh dear yahoo please help me! OMG THAT WAS SO LONG