My best friend is sitting with an girl on the bus who i USED to be friends with?

Sorry to hear that you're upset. I have been in similar situations when I was young. If your mom doesn't want you to hang out with her, there may be a reason. Perhaps next time you can take the initiative to say goodbye to them when you get off the bus, so that they know you're friendly. In the meantime, I hope that you will have some new, loyal friends who will grow up with you too. God bless!