As a teacher, I had to report similar for the reasons mentioned above.
I understand that both of you are worried that something that was said innocently has been viewed through the lens of it being sexual. I’m sure it’s a horrible time for you but unfortunately it really does have to be investigated.
“Put your hands down your trousers and play with yourself” Is a very odd thing to randomly come up with and just taking it at face value, it is concerning and not age appropriate. Based on what you have said, I have no doubt whatsoever that it was entirely innocent on your son’s behalf and that he has no idea that he just commanded another child to masturbate. But the fact remains, whether he knows the meaning of what he said or not, he did actually command another child to masturbate and it is a totally unacceptable thing to say.
Based on my experience, this will be handled sensitively because at the heart of this will be the fact the adults involved will be concerned for your son rather than angry at them. My first thought would be, where has this child heard this kind of language? Is there any possibility that he is being abused? Not that he is “in trouble” and that he should be disciplined harshly.
I’m sorry but your son has to have heard this phrase somewhere. Most likely another child. I don’t for a second believe he just pulled that exact phrase out of his head. Even if he can’t consciously remember, someone around him, maybe in the playground has said it. There’s also the possibility that he does remember who said it but is too afraid/loyal to tell anyone? He might fear getting them into trouble?
Hopefully soon, a line can be drawn under this. Kids some out with some absolute corkers in all innocence and as professionals we know this. But sadly there are some cases where a phrase like this can be indicative of something very bad happening to a child and so we have to investigate every case of language like this. It’s tedious for us and anxiety provoking for you and the children involved but until people stop doing awful things to kids we need to keep looking into any red flags that crop up.
Hopefully now the school can help your son come up with some tactics and coping methods in how to deal with annoying behaviour in others and that they can gently explain for him that he shouldn’t use language like the phrase he said.
From what you said he sounds like a sweet boy and you sound like a fantastic parent who is doing all the right things.