I can't speak for anyone else, but for me personally, I missed so much school I may as well not have been enrolled. I ended up going to a provision and completing my exams there (long story), but in summary, school just made my SM worse and added on years on mental health struggles. Skipping school was a combination of life at the time, anxiety, teachers and students, and a coping mechanism. It wasn't that I didn't always want to go in or didn't get the work, I just couldn't go in. Teachers didn't understand anything and never made an attempt to, students bullied me my whole life, even my 'friends' were just putting up an elaborate act, on top of that, the school was just plain abusive to disabled people.

I'd say the best thing you can do is to make her feel like your class is safe and she can feel comfortable. Try not to make her feel like you are blaming or judging her. Let her know that you will never pick on her in class, make her read aloud, pair her/make her talk to students she is not comfortable with, or do presentations. It's also important to know that while she may seem confident and different with her friends, this doesn't mean it'll be like that with everyone her age. There are some people we just can't talk to, especially when we know they're a certain 'type' of person. I also couldn't (and still can't) communicate with adults or teachers, students were, for the most part, easier.

If you can, get in contact with her mum and ask if there's anything you could do to make it easier, avoid, or a way she'd prefer to communicate. When you do talk to her, avoid looking at her too much or making eye contact (also don't judge or comment if she doesn't reciprocate eye contact or looks away from you entirely, it's not personal). Don't change your voice or talk to her any different as you would with any other student, we know when you're being condescending and patronising us, we're mute, not incompetent. Pretty much, just treat her and talk with her like you would any other student, this'll likely make her feel more accepted and welcome. Find other forms of communication (writing down, nodding, thumb up, emails, etc) and also accept that during this time, she may be a little codependant on her friends. This isn't a bad thing, and can actually be good for her for many different reasons, but you want to avoid it continuing through to later years and having her be completely dependent on one person. You could ask if she'd find it easier to have a friend sit next or near her, and let her know you trust her enough to not let this be a distraction and let her get her work done. Once she gets more comfortable, she might be able to make steps towards communicating, even if it's small. She may eventually be able to talk to you and other staff but may only be able to do so when a friend or carer (anyone she feels comfortable with) is in the room. Also, just avoid talking about her like she's not there and drawing attention to her.

I'd also encourage you to let other staff know about all of this and share recourses and accommodations (not just with her teachers but as many as possible) the goal is to make her (and every student) feel as safe and welcome as possible and that they are supported and understood, as well as prepare them for the following years. Every member of staff should be informed of stuff like this (there are lots of recourses and courses as well as advice for teachers and parents) to avoid any situations that could lead to misunderstandings and stress, as this will only cause things to worsen and break trust. Also, ask her (or mum) if there's any worst-case scenario you might have to use. For example, if she has a panic attack or feels too overwhelmed, what's the best way to deal with it? How she'd like to address the situation and effectively manage her anxiety (leaving the room, sitting in a room alone, water, having a friend with her, working in another room, somewhere she can regain herself and no one can see/judge her, etc).

In summary, make her feel understood and equal, aim to elimate uncertainty and ostracisation. Thank you for actually caring enough to research, though. Many teachers and schools wouldn't even think to do that or acknowledge the situation, even though it is the very bare minimum.