This will be my first f/f roleplay. I decided to give it a try and here's my idea.
Megan is off to college for the first time. She's placed in a room with a girl who has had many roommates who have all come up with some lame reason to be moved out of the room. The real reason is, the girl has tried to make each one a pet of hers.
Megan is sexually inexperienced and is petite in size. Her roommate would be bigger than her. She knows no one as she is attending a college 3 states away from home, so has no one she can turn to.
The girl can be in the room as Megan brings up her things, and the intimidation starts then. Megan will not be a willing participant at first until the girl breaks her will and pretty much isolates her from everyone by scaring off any new friends she makes. Eventually, Megan just gives up and accepts things. The girl can collar her if she likes.