YMCA swimming lesson incident

My son almost drowned in YMCA swimming lesson last week. There were 2 classes that night and only 1 instructor showed up. The instructor was with the other class when a kid in son's class pushed his head down the water and wouldn't let him to get out. My son eventually got free by himself, coughed out the water and cried. This is a Polliwog lession (beginners) for 6-7 year-olds.
I have demanded several times since then to speak to or meet with someone in charge in the branch but am going nowhere.
Is there any law or regulation that require proper supervision for kids' swimming lesson? This is not the first time when there are only 1 instructor for 2 classes. When the instructor's with 1 class, the other class was asked to 'swim on their own'.
I am in MA.

And did you file an assault complaint against the other child?

No. I don't even know the kid's name. Anyway, is it even possible to file an assault complain against a kid this young?

if you feel it is unsafe for your child to get lessons at the Y, why are you still taking him?