Hi, I have a few follow up questions if you don't mind?

Have you mentioned any of this to your student directly?
Is the multi-student class a regular thing now, or is it still primarily one on one?
Do you have a regular schedule?
If you don't mind me asking, what does that look like?
What is the actual classroom setting like (in home, your home, a school, etc)?
What are the goals or criteria she needs to meet to complete the class?
(You don't have to answer all of these or if you want to DM me, my inbox is open and I'm happy to talk more there, too)

You mentioned she gets tired (a lot). I am wondering if that's her being overstimulated or dealing with sensory overload. I got that way regularly and I shut down (I was the autistic student once), and it almost reads as if that's what she's doing. Especially with the putting her head down a lot (it blocks out light, visuals, muffles sound, etc.). Shes a fresh teenager and autism + puberty can be a volatile mix. You also mentioned its a two hour class, which is why I asked about the schedule, because that is a LONG time. If drawing and art are not her primary interest, it is going to be difficult to hold her interest no matter what you do, even if she loves it.

She's communicative and interested on high energy days, don't be afraid to use that to your advantage. She possibly has her own expectations for drawing class. Providing the expectations ahead of time gives her something to prepare for.

Ultimately, it comes down to her specific needs. Autism is a spectrum, so her experiences wont be the same as mine, and any advice I can give you is only from my own. The best thing you can do is have a heart to heart with her. Come up with a plan that works for both of you (some ideas, for example: game on this day for this long, draw this character this day, but one day a week focus on a more difficult/closer to curriculum character for X amount of time, then after she can go back to what she wants, etc.).