Masturbating is not the same as stealing a police car, nor is it bad for your health like smoking. When you describe the way you would approach the situation, why would you approach it like that? It sounds like the approach of a parent who loves their child and respects their right to bodily autonomy, but the response OP got from her mum was to act as if it's an abhorrent act to be controlled and stopped. Could you imagine doing that to your child and making them feel that way?

I often forget that that the US system is different, in my country they will only take the kids away as a last resort, maybe it would have been better to say tell someone, like a school counsellor, idk, I generally have a low tolerance for behavior that I see has parallels to abuse because of the experiences myself and the people close to me have had, and my reactions to such behavior are more severe

Edit: oh dear the comment got deleted

What's your take?

She shouts because wrong, she is probably used to getting her way by shouting others into submission, because even if she's wrong if she's the most aggressive and you comply she feels like she's won

It's outside of the mums control, therefore bad

This advice is for dealing with a reasonable person, and ngl, OPs mum sounds like a bit of a narcissist and sounds like she sees OP as her property with no right to privacy

That sounds like abusive behavior to me, your belongings, your clothes, your space, aren't yours, they're hers; and honestly if your grandparents are worse and this doesn't stop, seriously consider contacting child protective services