Might be a good idea to put your concerns in writing. Sometimes it's down to the cohort, some are worse than others. (The year below mine, at school, were notoriously quite awful!
) Boys being too much like boys, perhaps???
I know 3 boys in the current yr7; with 2 of them, I would be very very surprised to find they were doing this, whilst the other, a joker, may be influenced unfortunately. If it's as severe as you believe, a letter to the housemaster would not be a bad idea, stating that you are concerned and even after you know he has spoken to them, have heard it has not abated and are extremely worried about the ramifications.
It's all rather disappointing