My two cents on two of the debates here: The MC doesn't mention the prostitution ring to Sage because if
a, he is not ordering from the restaurant - he is not interested in these things and will not interfere with it, also he will then never figure out the size of this ring and which girls are involved, so why would he mention this to Sage?
b, he is a costumer of the restaurant, which means he is enjoying the situation, why would he talk about it to Sage, who clearly has no idea, talking about this to her could mean the end of fucking HOTs for money.
I just replayed from the beginning and in the preps party Sage talks a lot about Burke of course without naming him. "my father is downstairs and i'm avoiding him" etc. Since she wants to avoid contact with her parents she herself doesn't mention this to MC and also why could this not be an information that noone would share with him? Burke is not teaching MC-s class, Sage is not anywhere near in school to MC and all the other freshies. This info could very well have stayed private, also if other students also understand that Sage doesn't like this relationship. I don't see this AT ALL a writing error.
I´m not sure why the MC don´t tell about the restaurant to Sage. She´s the president of the HOTs and, if the "restaurant" comes to light, Sage´s good name and the other HOTs that are not into the bussiness is going to be affected too.
I think that, if the MC is not a "costumer of the restaurant", the dialogue between Quinn and the MC when she makes her offer and the MC rejects it should have been something like this one:
"- MC: This is another of your mental games, isn´t it, Quinn? If I say I´m interested you will make fun of me calling me pervert. Well, I don´t pay for sex, so I´m not interested."
"- Quinn: OK, yes, you got me, it´s a joke. Now out of my sight, pervert."
That way, the MC would not mention that to Sage or others because he believes it´s just a joke of her, or at least he´s not sure if the restaurant is real or not.
If the MC is a costumer of the restaurant, as you say, he don´t talk because he´s enjoying the situation.
About Sage knowing about the restaurant, that has been debated in other comments... why? She doesn´t get money from that business, she don´t need it, and I don´t think she would be happy knowing that her own foster father is a client. And she would know if she were into the ring. She just don´t know because her mind has been absorbed by her problems with Chad and her studies for her graduation, and has left most of the business of the HOTs in Quinn´s hands.