I agree with PP - your expectations that your tiny daughter is not up to this is what is causing some (alot?) if the issue here. You view her as small, vulnerable, shy, not able to cope, a victim of others being bigger/tougher.
So your anxiety filters down to her and her behaviours.
You need to hoik your big-adult-pants up and change your stance - at least externally.
So no "Poor you being abandoned on Friday after school" talk. Leave her to talk to her friend.
Stop giving her a lift to school in the first place.
Change your conversations about Friday night to "Wow, you dealt with it so well. I know it was scary - but you came home on your own. Showed me how well you can cope/adapt. I am really proud of you." "Wow those Yr 11s must have been a bit scary - well done for getting home and telling me about it. It must have been tough - but look how well you did even though you were upset." (Obviously amended depending on the result - but find some positives/bravery to build her resilience - not sympathy to reinforce her weakness).
Accentuate how well she is coping, how well she can/will continue to cope.
You are babying her - and it will not benefit her.
And I say this as the mother of a new Year 7 DS who is also the size of a Yr9 and only for the first time ever met friends in the park last night. My DS is disorganised, in no rush to grow up (prefers lego to Fortnite) and before this term never walked to school before (couldn't get into the local schools so I had to drive him). We breezily expected him to cope (terrified inside) and he has. though he still takes a fucking age to get ready in the mornings