I'm having something similar here with a Y11 15 year old. I think the first thing to remember is that they, and we, have been through an extraordinary year unlike anything we've ever known before. I've also felt overwhelmed by it.
The trouble is, for the Y11s the logical arguments for them being in school don't really hold up. They know perfectly well they can study from home. They've basically finished their courses and are mainly revising now anyway. And to be honest, having been in an office yesterday for the first time in over a year, it really freaked me out being around other people for two hours, so god only knows what it's like for them in school, and particularly what it was like during the autumn when they were all crammed in with no testing and no masks in class. I've got a feeling I would be refusing to go to school too.
My ds seems to be able to go in one week and then not the next. His anxiety does manifest as physical symptoms - upset stomach and urinary urgency, both absolutely impossible to manage in school when they have to wait for a duty supervisor to take them to the loo and wait for them and then escort them back again. So every morning he doesn't go in, he immediately emails his teachers for the day's work and every single piece gets done. He did an assessment at home this week, supervised by his dad.
I've spoken to his head of year about it today, and we have an appointment with a fantastic GP on Monday. My view is that he needs to do what's required to provide the evidence for his grades, but basically that's it. I'm hoping the school will continue to be sympathetic and then that things will be more normal for sixth form.
So - you definitely aren't alone. My ds never had any attendance issues before this year, and when he does go in he seems perfectly happy to be there. I think we just have to try to get them through the next few months the best we can.