Season 21, Episode 15
A new girl at Springfield Elementary falls in love with Bart, but he gets into trouble when the girls parents discover he has kissed her and the school has to adopt a no touching policy between pupil and teacher.
See here for more details.
There is a sequence which parodies some famous on-screen kisses, which include:
From Here To Eternity, Gone With The Wind, Lady And The Tramp, Planet Of The Apes, On Golden Pond, Ghost, Spider-Man, Star Trek, The Public Enemy, WALL-E, Alien³, The Godfather Part II.
[see CREDITS for the screenshots]