What I would suggest doing is to have a conversation with B and C during your lunch break or whenever at school about the situation. Just directly address it and tell them how things have been between you and D but also mention how you're generally feeling about all of this and ask B and C if you may have done anything that has annoyed or upset them, I know it can be difficult to directly address things especially when it may feel like it's 2 or 3 vs 1, but it's better than leaving it as one big guessing game and then having them find out about how you feel from someone else etc, whatever the situation is I hope it works out for you, sometimes people change as we get older and sometimes that means changing crowds but what we all struggle to realise is that sometimes it can be for the complete better. I'm so bad when it comes to dealing with change, when year 12 started, I hated it so so much all because I just kept missing secondary school years, but now that I've gotten used to things and have made friends (all my old friends left my current school) I'm happy. The situation you have with B and C may just be a big misunderstanding or they may be bad company for you, which in that case, just remember it's always fine to cut out fake friends etc, One of my friends from my year 11 friend group has stopped talking to all of us and just cut us off even though she's at the same college as some of the same group, she hasn't given a reason which is stupid but none of us are fussed because if that's the type of person she is then we can see that we're better off without toxic company like that.