"You can't place people into a course which has prerequisites and instructor approval if the student has not met both standards for admission. I know it's difficult to make this work, but the needs of the other (XX) members of my varsity ensemble are not being adequately met while I have to effectively baby-sit these twelve students who were improperly placed."
or, if you really want to scorch some earth:
"If these were non-ESL students placed accidentally into an ESL class, would this be as difficult?" "Students moving from a PAP class to a regular level class?" Or, my favorite, "Students in athletics who didn't have a physical?"