Sorry I hadn't realised this post would be so long! Thanks for reading if you manage to get through it 
My DS (5 years old) has asd, my family and friends know and so do the teachers at his school, but the other parents at his school or his friends parents didn't know about it.
There is another little boy in ds's class with SN who hits quite a lot (I'll call him x), I was talking about how my ds was very upset after school recently as one boy had bit another and he was worried about them both as they are his best friends. One of the parents then asked, oh is that x? Then they started talking about how he is a really bad boy, he hits other kids too and a lot of the other parents had got together and talked about it saying they should do something about it. They then started talking about how they had never seen his parents and they must be junkies (heroin users) since their son was so bad.
I have seen their parents, they pick x up from school so I'm not sure how the other parents haven't seen them, they aren't 'junkies' either, I think they are quite nice. This made me very angry tbh and I said that we don't know the whole story and noone knows everyones situation, and explained that my DS has autism and he hits a lot when he can't cope with a situation, particularly when he first started school he hit quiite a lot.
They were shocked and said things like "Oh really? I never knew", "You'd never be able to tell", etc. I felt that I had to stand up for x and his parents as this wasn't just a one off, the conversations about x and his parents are quite frequent - yes he shouldn't hit but tbh I don't think this school offers much support for those with sn and I am moving my child because of this - but I don't think talking this way about him or his parents was the right thing to do either.
But now I feel guilty that I shared my ds's difficulties and am not sure if that was the right thing to do either? Ds knows that he is on the spectrum, well as far as a 5yr old understands, so won't be surprised if another child mentions it but I'm not sure if it was my place to discuss it with his friends parents or if he should be the one to tell his friends when he is older.