Sorry to hear about a bad experience, you are probably feeling very sad.
There are too many variables in when to let go and give freedom, but I think you do have to try and prepare kids for the real world, warts and all.
I would try and turn the negative into a positive, praise your DD and her friends they coped with a very difficult situation and did the right thing, I would give them lots of praise and encourage them to try again.
When DS1 was in year 4 he wanted to go on his own to the park, so I had to follow thirty yards behind and sit on a bench at the park and read a newspaper. Today, nearly Y9, we walked through Reading town centre and he insisted on walking ten yards behind me, eventually I could not resist it and so turned round and said in a loud voice, "are you following me" just to embarrass him.
I think he saw the funny side of it.
Concentrate on the transition to the new school, the freedom and independence will follow. Hopefully soon you and DD will be able to make light of it and share a laugh about those poor sad teenagers that go around being a nuisance.