Co-operate and put aside your anger - at least one of these children is troubled and has had to grow up too fast. Bear in mind that my DD2 (7) watched Princess Diary a few weeks ago and exclaimed 'euurgh, Mum, they're sucking each other's lips off....' - it was the first time she'd seen a 'proper' kiss. Oral sex is not even in her universe, let alone her world. It just doesn't exist.
There are three children involved in this story and the school has to observe confidentiality and only discuss your child with you.
So the school have a duty to pass on the information that they have collected - that child a alleges that she witnessed DS1 receiving oral sex from girl x.
Personally, I don't think you should have been told who had alleged this activity. Even if the child is known for 'causing trouble' you shouldn't know that was the origin of the situation and it definitely shouldn't affect how this is managed.
The professionals now need to look at 3 scenarios:
a) Child a is troubled/has been exposed to sexual activity and needs some support.
b) Girl x is troubled/has been exposed to sexual activity and needs support.
DS1 has been assaulted (don't forget criminal responsibility kicks in at 10, although dealt with differently in most cases) so needs support.
Child a was absolutely right to report what they saw, whatever their motive for doing so.
c) DS1 is troubled/has been exposed to sexual activity and needs support.
Girl x has been assaulted, so needs support.
Child a was absolutely right to report what they saw, whatever their motive for doing so.
Whichever scenario has occurred, at least one child needs support. In fact, if it is a false allegation, your DS1 and the girl will need support anyway to overcome the impact of that.