Bristol Palin is officially persona non grata at Washington U in St. Louis -- in fact, when students found out she was giving a talk on teen pregnancy, they threatened to protest ... so B.P. pulled out.
The talk was supposed to go down next month during the University's "Sexual Responsibility Week" -- but a rep for the school tells TMZ, it ain't gonna happen ... "because of the growing controversy among undergraduates over the decision to pay for [Bristol's] talk with student-generated funds."
Bristol -- a spokesperson for the prevention of teen pregnancy -- was supposed to give a brief lecture on abstinence. According to the rep, it was a mutual decision between Bristol and the University. What a tripp.