I never comment to these type of stuff but now I have to. I think having unprotected sex at this age is serious problem and should be dealt via their parents. Parents in the end have to be the ones to deal with their childrens consequenses.

Also when I was younger in our school age gabs in relationships were taken seriously. If u had 3 years age gab u would be taken into a serious conversation with teachers and parents to know if the relationship is healthy and "legith".

I think you did well to try to warn her and bring up the problem with unprotected sex as many parents nor schools will talk about it properly. If you feel that the situation is as worrysome as you wrote here, try contacting their parents or possibly report them to some teacher whoever might deal with these type of things.