I (we) am only being being hard on you because it really is a very serious subject to broach on.
There is always a very, very good reason that lingering suspicions of his dubious behaviour hasn't been answered with any charges - the man is more than likely innocent. As has been mentioned before, schoolgirls do have an incredible capacity to make up the most vile things about their subject of loathing, especially in school as they do not fully understand the ramifications of such accusations.
I am in no way saying your girlfriend is a liar; just that these sorts of hearsay really are common and they really do easily paint teachers in a harsh light that they often totally do not deserve. I know of a teacher myself who's had this stigma for years simply because of his appearance which some spiteful and hateful girls didn't take a liking to. We all know
those school kids.
The police do operate to a level that they are very very good at checking out said allegations and putting genuine paedophiles away, but the fact is that this man hasn't even been the target of any investigations strongly shine him in a light that shows he is innocent, whatever reputation he may hold.
All I am saying is, keep a level head and tread lightly; he may be a paedophile.... or he may not be at all, which is the overwhelmingly most likely scenario here.