First of all, the age you started meeting is quite polemic...

Apart from that, I am teacher too and being alone with teenagers (both genders) and I have done nothing weird... there should be nothing wrong in seeing a male teacher teaching a female student... you should not feel bad for not seeing it because it is not wrong per se...

What would I do? It is quite easy (but painful):

-If you are sure he did it, break with him

-If you are sure, he did not do it, support him

-If you doubt about him, break with him... if he is guilty, you dont deserve him. If he is inoccent, he doesn't deserve you. I know my partner pretty well and if he (we are gay) was accused to be with a minor, I would not believe it at first.