Ask her what a mentor is.

You hesitantly ask what a mentor is and the librarian seems a bit confused. "Silly you." She says "It's a member of the faculty that own you. Or put more delicately, someone who guides you in exchange of your services. Being the School Whore you have the option of choosing your own mentor, but if you do not choose the faculty duel over you at the end of the week." You seem a bit confused. Why have nobody told you this? You look at the librarian rather perplexed.

"Don't look like that. It is usually the headmaster that wins the duels... And he too is the one that is suppose to inform you of this." She laugh a little at what clearly is a broken system. "Anyway, i have never won the right of the School Whore so i would be delighted for you to become my little pet. I will even give you acces to the forbidden sextion." She blows you a kiss as she gives you a wink.