Let's go in-depth to the scene and clarify every doubt you may have:
1. You wake up. You decide if you want to become a DIK or not: Yes; No. Cumpetition starts.
2. Quinn says she called dibs on you. She starts to grind on you and provokes you by insulting Maya. You choose if you defend her or not.
3. Quinn continues to talk, her words varying according to what you chose in the previous choice, and grinds on you until the third student loses.
4. After that, Tommy reveals the true purpose of the cumpetition; you and Derek are now maggot brothers and both of you will have to wear a condom and the loser will have to eat it.
5. The condoms are not seen being put, but, relying on the text, we can assume they are. Quinn continues to ride MC; he starts to count Mississipi's; Quinn asks Riona to cover them.
6. The moment where Quinn removes the condom is not seen either, but we know when it takes place by the onomatopoeia *
Sshhhllllpp*.
7. You decide if you make her stop or let her continue.
7. a) You chose to make her stop; MC will claim he came, and Tommy declares Derek to be the winner. Quinn shoves condom down MC's throat.
7. b) You let her continue, Quinn inserts MC's dick into her vagina, thus fucking him raw, but soon Derek cums and you win the cumpetition.
I hope everything is clarified now.