Trip with the Teacher (1975)

A pretty young teacher and four of her pretty students take a minibus to some Navajo ruins, little suspecting another kind of ruin awaits. A trio of bikers begin to flirt with the gals, and when their bus breaks down, they helpfully tow the vehicle to a deserted shack. The chief biker, Al, chews up the scenery, kills folks, and rips off the teacher's clothes before raping her. The poor schoolgirls are also in for their share of abuse.