I sympathise, but like others believe that you have to go through official channels and get the school to follow this up. I think you have the right to be kept informed as to what the school is doing and about any progress they are making.
My cousin found out that her daughter had been exposed to p*rn at the age of around 9/10 by a (female) school friend of the same age. The first my cousin knew of it was via the discovery of some graphic drawings her daughter had made after a play date with the school friend at home. Shortly after this, she discovered that her daughter had seen p*rn on her school friend's iPad at the school friend's house. She confronted the girl's mother, who immediately blamed her ex-husband. It turned out that the mother's ex had a p*rn and s*x addiction problem, which was the main reason they had divorced. When he had the children every other week end, he wasn't very careful about hiding what he had been looking at on his computer - and that's how it all started. But moreover, as a newly 'single' mother with a full-on career, she had given her daughter no less than 2 iPads and a mobile 'phone by the age of 9 (!!). Maybe as a way of keeping her occupied during the mother's long absences whilst at work, maybe bought as a means to assuage guilt about what had happened in the girl's life - one can only speculate. But the net effect had some undesirable consequences.
Luckily, with some extra parental help and supervision, it doesn't seem as though the experience has had any lasting negative effect on my cousin's daughter, who has turned into a wonderful, happy and healthy young lady.