of course he should and teachers should join thier students too- obviously, if the kids will get overly rowdy then not but otherwise, i think its a great way for a rebbe to show his class that hes not just capable of spouting lectures but can actually enjoy himself too!
It’s a great way to bond with his students, while at the same time keeping a close eye on them. It tells them that their rebbie is a human being, who can spend enjoyable time with them outside of the learning environment. If he has a good voice and can play guitar, noch besser.