Just to add that the school might try to guilt the OP into backing down because of the boy's learning difficulties. (The school should not discuss that with the OP, but I've known schools to try to use guilt to fob off parents.)
As already discussed, the boy's difficulties are not the OP's problem. However, a tactic that I used in the past - when someone tried to suggest to me that a boy behaving in a sexualised manner might not understand the impact of his actions was as follows: I agreed that the boy concerned might not understand what he was doing, but pointed out that the boy needed to be given help in order to avoid getting himself into more serious trouble in the future.
Having said that, the OP's focus absolutely should be on safeguarding her daughter. I'm just mentioning the above because school senior management (in my experience) often focuses on 'high tariff' pupils rather than those affected by their actions and I want the OP to be prepared.
If the school is stupid enough to refer to the boy's learning difficulties, the OP can express sympathy as suggested above but should be firm in stating that her daughter is at school in order to be educated in a safe environment and not as a socialisation tool for the boy. The emphasis for the OP is on what the school is going to do to safeguard her daughter and to deal with her trauma.
I'm assuming that the OP will be mentioning that she heard about the incident from another parent. Therefore, it's obvious that this is of concern to other parents.
As a middle manager, I once had to deal with a situation where a boy with profound difficulties was masturbating in class. The school was being pressurised by the Education Authority. The EA was being pressurised by the boy's parents who wanted him in a mainstream class when it was obvious that we could not cope with his needs.
I wrote a factual report describing what had happened. The incident was mitigated slightly by the fact that all the girls were elsewhere that day. (I was in class, as was the boy's assistant.) However, the other boys were aware of what had happened and had commented.
I made a point of stating that I was very concerned at the possible impact of a repetition of the incident when the girls were in class and pointed out that the EA risked facing legal action from parents.
In the end, the parents claimed that the boy had only been scratching, but agreed that he could return to the specialist environment where he'd previously been educated.
Before I retired, teachers had to report worrying incidents to the school Child Protection Officer (one of our deputes). He would then decide whether to call in SS or the police. (The latter happened when an adult contacted two of our pupils.)
Would be worth asking what the procedure is at your daughter's school, OP. If the boy in your daughter's class is exhibiting sexualised behaviour, it can be a sign of abuse.
Again, I'll emphasise that your concern is for your daughter. The school should not give you information about any referrals that they've made about the boy, but it won't go amiss if you show knowledge of how things should be dealt with.
Schools are scared of "pushy" [translation: "assertive"] parents.