A bunch of parents brought their kids to see Zac Efron in his G-rated High School songfest -- they got full frontal nudity instead.



It all went down in God-fearing Utah -- where managers at a South Jordan movie house decided to move a screening of "High School Musical 3" to one of its bigger theaters at the last minute.

But here's the fun part: according to the Deseret News, someone forgot to swap out the movie reel on the projector -- which was set to show the R-Rated flick called "Sex Drive."

Suddenly, it was lights, camera, and all sorts of sexually explicit material.