I just never play or watch the same thing twice if I can help it.Ok, so, I'm genuinely curious here. If you forgot that much, why not re-read the whole VN from the start?
As for the answer to your question... Zack is the star athlete at their University and the guy in charge wants him to stay that way. He old dude basically tries to force the couple to pass him with fake grades, and they refuse and instead try to tutor him. Zack doesn't really give a shit about anything except having fun, so while he initially seems to go along with the tutoring, it's because he thinks Sophie's hot and he wants to do lewd stuff with her. Things escalate if she gives in to his demands, or go into the good for Felix ending if she refuses.
BTW, how did it start? The sexual things. Why did she initially go along with it?