Nan - apologies for the delay.
My Dd's experience of returning to school (to the teen life she wanted) sounds similar to yours. She didn't have a close knit group of friends, she was two years older than them and soon noticed that maturity gap. Her own year group were supportive of her ED whereas this new group didn't really know/connect so there was lots of diet based chat in the common room which triggered DD.
At the same time, she struggled with the workload, some of that related to the sheer level of effort she had to put in to concentrate due her severely depleted cognitive function and also because she was comparing her ability to before she became too ill. She was getting stressed about reaching the levels she was able to achieve previously (a-levels were her thing so much more than gcses - she'd found her niche)
That combo meant the school environment became toxic and the anorexia saw its chance and grabbed it - with some relapses. Eventually dd thought it through and said she wanted to leave. I surprised myself with how quickly I came round to her way of thinking (although I still had private 'woe is me' moments about the future she wasn't accessing)
Two years later she's happier, healthier, switched on, interested in the world, has two jobs (personal trainer and working for charity which is an umbrella body for other local charities), she's using her brain, even in early days of new job she's coming up with ideas and executing them, experiencing new levels of independence.
I share all that background because for Dd, it feels like there were two sliding doors moments - one was forced upon her when the anorexia took control. A pathway to one future was shut off then. The second sliding door was when she decided, having tried to pick up the original/initial path way, that she knew that future wasn't for her and so picked a new pathway - I guess we don't know that lies down the road but for the moment she's on firmer ground underfoot that she's been on in years.
I guess, I'm saying - how important is it for Dd to persevere with trying to reconnect with the footsteps she never got make on her original pathway or would it be better to take a little stroll elsewhere - connect with new friends her age through different hobbies, volunteering etc. Dd had a crazy job in a bar/restaurant with loads of people her own age for about a year and it gave her a new group of friends, socialising opportunities. Then she decided it wasn't the healthiest environment so she moved to retail where she worked with older colleagues who were her mother hen and now she's where she is. Although she was happy to leave both environments, they did both give her something she needed socially/emotionally etc even if only for 9/12 months...
Given the 'focus' your DD has put on the 'getting back to where I was' as a drive for leaving the unit, I can imagine she (and you) may not want to have the "is this right" conversation yet with the fear that without that drive the ED will slip back in. But perhaps doing some future visualisation activity may help - not so much of "what am I doing" but more of "what matters to me" - We did similar exercises with Dd about how she wanted to feel - so 'happy' 'able to be spontaneous' 'strong' etc etc and that could be used to open up a conversation of how you could be those things in different settings...
I've rambled but most of all sending love!