Ain't a professor, Im still a grad student, defending my dissertation in three weeks. But I teach my own classes over the summer, and teach labs during the year.
It happens all the time breh, especially to TAs because they're closer in age. Its almost never them trying to get better grades, they're just attracted by the position of authority (which is super LOL, considering the lack of authority you actually have as a TA), plus bitches love smart dudes.
My advice: Dont do it. But if you're got to wait until the quarter is over, so you're not technically in a position of power, and then tear it up. You got to be careful though, cuz a crazy bitch can ruin your career quick fast in a hurry.