You see they feel 'normal' when school isn't on. That's why you see them brighter at weekends.
My DD dropped out of school just after the start of her final year ages 16. She stayed in bed til early afternoon, but then I noticed would get a bit brighter then when the school 'day' was over.
When the exams were over she was happier because she said 'everyone is the same now'. Meaning 'I don't feel different and isolated because everyone at school and I'm not'.
It's like being on the Dole. At the weekend you are normal but come Monday you are a failure and different again.
She had been going along with stuff for years, then when the senior cycle hit and suddenly the 'future' as an adult loomed it just all came tumbling down.
She never set foot in a classroom again but she did go in and do her exams. I had to do a lot of liaison with the school.
With the exams I said 'do you think you will regret not doing them more than you might regret doing them? But it's your decision'.
Then she did a year long art course. Now she's gone to college and as independent as anyone her age.
So take heart, the school environment doesn't suit some people and it's overwhelming. But that doesn't mean they'll never come good.
But as a parent there is grief involved. Letting go the expected 'normal' path of school.
Especially if, like me, you are the anxious rule follower type, someone not going to school was very hard to get comfortable with.
I went to therapy myself when that was going on. Because I needed to not take things personally and find somewhere to explore my own anxiety and worst fears.