Thanks for the replies and support.
Well we had another class tonight and unfortunatly they misbehaved again. Again, they couldn't even be sensible before the class started. I went to the changing room and one of the girls was throwing toilet roll over the cubicle to where the other girl was, both finding it hilarious.
A little later, we were nearly ready to begin, we had a few minutes before we were due to start. One of the ladies that always comes to the classes asked if she had time to quickly go to the toilet. Of course I said yes and added that we wont be starting for a few minutes. One of the silly girls then shouted out 'what if she is going for a poo' and again they both found it very funny. I just thought, how immature.
During the class there was some giggling etc coming from them, so I asked one of them to move so they were not together. Unfortunatly that didn't work. I hope I wasn't too harsh but I ended up sending them both to the back to the changing room.
Didn't expect this kind of thing in adult classes...