Originating in the Harry Potter series by J.K. Rowling. Was a play on Professor Slughorn's name and his continual favoritism of students.
(Used usually after the words "in" or "part of".)
To be a teacher's pet without trying or even wanting to be a favorite. Most people in the "Slug Club" are called upon to answer questions, do unnecessary tasks and will receive numerous compliments from their teacher.
The student will be thoroughly embarrassed, which the teacher remains oblivious to.
Student 1: Oh no! Did you see how Professor kept pointing out how perfect my potions were today? How humiliating!
Student 2: Welcome o the Slug Club, mate.