My autistic son came home from school with marks on him

So a little background, my son is 6. He was diagnosed with moderate to severe autism at 18 months. He is non verbal. His speech and expressive language have improved so much with lots of speech therapy but he still only uses one word utterances to communicate. Last year for his kindergarten year we stayed virtual. This year for first grade we had no choice but to send him back to school as the district eliminated the virtual option.

His first day back he of course was very apprehensive because he has never been in a school building before. His schools policy is that parents cannot enter the school itself so the students are met at the gate by teachers and teaching assistants who walk the kids to class. I met his teacher briefly on the first day and she looked visibly annoyed that my son was so upset but assured me he would be fine and told me to leave fast because if I stayed too long it would make it harder on him.

I picked him up after school and he looked so happy to see me! He of course can’t tell me about his day though. We get home and he wants to change into his pjs so we do and when I take off his shirt I notice a nail mark on his shoulder. And not just a scratch I mean this is a cut that looks like it bled and is starting to scab over. I’m heated so I call his teacher and she assures me it’s not an adult finger nail mark, that another kid in his class pinched him and she apologized.

Day 2 of school, I pick him up and I notice right away that he has ANOTHER adult sized nail mark (cut) that looks worse than the first day, on his arm. I call his teacher and she doesn’t answer my call, I email and she doesn’t answer my email either.

On day 3 (today) I keep him home because I want to get to the bottom of this before he goes back. They mark him absent of course and the school office calls me for a reason for the absence. I tell them why I’m keeping him home and they tell me that it will be an unexcused absence and he’s only allowed 4 of those per year so I need to send him back tomorrow regardless of if the teacher responds. Now I just feel completely uneasy sending him back to that school at all and think I’m going to pull him out and homeschool. I mentioned this to the woman on the phone and she said I couldn’t because my son has an IEP. What do I do here? I refuse to send my son back when something is clearly happening to him. He shouldn’t be bleeding two days in a row from clear ADULT sized nail marks on his skin. They 100% happened at school and not at home. What would you do? Advice appreciated.