Oh Hedgehog... have a hug. You know, if you want. I do not condone forcing hugs onto people.
It probably would have been better to say to your DD you needed to check with school first, you'd give them a ring today & a decision could be made for tomorrow - reminding her how supportive they'd been previously to assuage any anxieties she might have about them saying no. If they DO say no, would your GP be willing to write a note explaining why it's important she be allowed to wear cover-up?
However, it's not the actual end of the world that you didn't. You didn't let her go in with it trowelled on & in her own clothes (assuming school has a uniform) for good measure. You had to make a decision v quickly & you made one that let your DD face school feeling just a fraction more confident.
You've had some fab advice on cover-up here (so much so that I might even investigate myself - with winter coming steroid!skin is on the horizon once more...) so perhaps (if you can afford it) you can take some time with her to shop for some new make-up & then practice using it so it looks as subtle as possible but she gets the cover-up she wants.
I didn't have a diagnosis for my "main" disability when I was at school. In fact, I still struggle slightly with the idea constant pain isn't normal. I no longer constantly feel sick, but the idea most people's bodies Just Do That is still a bit weird. (I just thought I was being really nesh & needed to woman up so I never complained. I would just bang dislocated joints back into place & carry on. Fail self, so much fail...) However, I remember only too well the feeling my body was outside my control. To be honest, it still feels that way.
Part of the reason I self-harmed as a teenager was to gain a sense of control over my body - and one facet of my anorexia is that same search for SOMETHING I can control about my body. I don't think anyone could disagree that in comparison make-up in contravention of school rules is a fantastic option. It's really hard to convey that feeling of utter desperation & of being imprisoned in a faulty physical form.
Mostly I startle medics bent on holding a pity party for me with my Positive Attitude. Because it pretty much is Be Positive Or Die. But every now & then it does all get a bit much & I will have a bit of a woesplurgle about It's Not Fair and I Am Scared (etcetcblah) - OP's DD will need to do the same from time to time. I'd be much more worried if she was going full Cousin Helen 24/7/365.
Not that you're obliged to say, clearly, OP, but is your daughter eligible for surgical intervention for her scoliosis? Mine is v mild (just in an awkward spot so accelerating damage) but one of my Guides had the surgery a few years ago & it's made a huge difference for her.