A couple of things. I would like to add. As stated in Red Hot Nirvanas solution, you do need to save all classmates and then beat Killbilly for this achievement. I played first through normal and missed a couple of classmates. I went back on easy and rescued the ones that I was missing, then went and beat Killbilly on easy, skipped credits and the achievement did unlock for me.
So, if you are like me and missed some classmates, you can go back through stage select on easy, rescue the ones you need, (in pause menu hit x and it will show you which stage and how many classmates you need). Then go beat Killbilly and it should unlock.