DD has been going to town on the bus, with 1 or more friends, since the summer after primary school. They browse Top Shop, if anyone has any money they spend it in Primark or Starbucks, and buy sweets or cookies.
DD and her friends are all very sensible, all 11/12, although also capable of being entirely silly and hysterical.
My worries are:
older children being nasty/ trying to take their money or phones;
a 'friend of a friend' suggesting something like shoplifting, and them getting dragged into it.
However, they are always in busy areas, they are always together, they have enough sense and confidence to talk each other out of doing anything really wrong (as opposed to foolish), and they really are old enough to start dealing with difficult situations and peer pressure.
I did have a talk with DD about standing up to peer pressure before I let her go, and of course this is an issue at school as well.