There are several issues (societal and legal ) that contribute here.
First of all, the minimising of sexual offences in general, even more so when it involves children. They're too young, they're just boyfriend and girlfriend, boys will be boys etc.
Secondly, you have "public interest " prosecutions, a lot of incidents barely make it to trial,much less get a guilty verdict. Even with a guilty, sentences are lenient and the focus is on "rehabilitation ". The aim is to get offenders back on the streets and be productive members of the community. With a child that aim is even more important.
Then discrimination, various rights etc. come into play. Including the right to an education. So those kids have to go to a school. Once there, several rules,policies etc. tie teachers' hands.. be it by the system or SLT. You can't warn other students, you can't prevent that child from having a relationship with someone else, you can treat them differently, make comments etc. Safest avenue possible is report any concerns higher up.Of course,that's assuming they do care, which wasn't often the case in the past.
I have a friend who is a TA and was sexually assaulted by a student. It was all swept under the carpet until she had a breakdown in school. Due to the age and vulnerability of the student police were never an option.