IF Zac has such a close relationship with Alyssa I think he shouldn't bother telling her, the last thing he needs is to tell his mom that his teacher, his friend's mom and friend of his mom is hot, opened her legs or bent over in class and saw her hard nipples poking through her blouse...that would make for a terrible "pillow talk" instead of focusing on her as he should hehe.
As for him telling Dylan, that would also be stupid of him, better to be around whenever Dylan hears the news from around the school by other students instead of him and the smart thing would be to show support to Dylan and help him out in finding the truth about Sophia's behavior, that way all relationships/friendships would remain intact.
Though I agree, he should have heard something by now or the first task. I don't know nor think we'll get to the 12th task and maybe end the blackmail sooner...but "how?" that's the intriguing question.