The masking at school could be what's causing or at least heavily contributing to a huge problem at home.
Have you read about the 'coke bottle effect'?
It sounds like perhaps she's fawning at school because she feels unsafe there, she's using all of her energy, complying and people pleasing and coming home and letting it all out there.
Lots of neurodivergent people struggle with sleeping patterns so that is another box ticked.
My daughter didn't get diagnosed until she was 16. I didn't know much about autism until she was 13 which is when she got referred for assessment. When I look back some of the signs I would have picked up on had I known about them:
Verbal stimming- screeching very loudly when entering a quiet room.
Echolalia - repeating phrases either sentences someone has just said or repeating something off a TV show or video she had seen, may have been weeks ago over and over.
Didn't like tags on clothes. Had to have her shoes tied a particular way.. As tight as possible and needed many pairs of socks so her shoes felt really tight. Sticks to the same safe comfy clothes
Needing to sit in exactly the same spot at the table/in the car.
Sensory seeking, loved anything squishy, slippy, gooey (my son is the opposite to this.. Hates sticky, slippy, gooey... He's also autistic)
Small group of safe foods that she will eat.
Super sensitive to smell.
Very sensitive to temperature, especially heat and can become extremely dysregulated when hot.
Always moving, often hanging upside down from or jumping from the sofa or bed, jiggling her leg constantly while sitting down 'quietly'
Struggles to manage emotions, can go from perfectly fine to uncontrollable in what seems like seconds.
That's some of the things I can think of.
Just a thought, that maybe she finds the bath calming? Hence the struggle to get out? I agree with the other poster who said to not tell her she has to get out, rather offer choices so she feels in control. There's no real harm in adjusting bathtime if you can, maybe start it a little earlier and give her a little longer?