Dear Shona
You have a number of choices here, but the main two would probably be to get in there first before the school ring your parents and tell them your version of events.
The second choice is to wait and see whether the school do in fact call your parents because you use the term they have threatened to call your parents so there is an element of uncertainty. This could mean that you go ahead and tell your parents and then after all that stress the school don't ring.
The positives about you telling your parents first is that they get your side of things and also they should appreciate that you are choosing to tell them rather than wait for the school to contact them.
When you do talk to them at whatever stage, you should think about being honest like you have been in your email. Stating that you felt this would be a way of you fitting in with the cool crowd but that now you realise that this was not the best plan as it has just got you into trouble.
It sounds as though you don't have a very good self image if you describe yourself as the school loser. I am wondering whether this is how you see yourself or whether this is what people have actually said to you. Either way, messing around is probably not the way to get included in the cool crowd on a permanent basis and in this instance it sounds like you were the one who got in trouble anyway.
We tend to choose our friends by common interests or likes and dislikes, music for example. I wonder how much you have in common with this so called cool crowd or whether that is there attraction, just that they have this tag of being cool. Maybe there are less exciting people in school who have more in common with you and who would probably make more constant friends who would not necessitate your getting into bad behaviour in order to fit in.
You need to like yourself in order to be liked and that is something you could call ChildLine about on 0800 1111 if you felt you needed to talk to someone outside your family and friends network.