Again, the main thing is not knowing their lines. This play is a farce... and that means there is timing. When one actor doesn't know, it skips or worse drags... like a tape in a tape player... you know it is dragging and it upsets your rhythm. Same thing with a play. The audience knows it is dragging... and it drags because not only do you have to feed lines, but you have to convince the actor on stage it really is their line so any momentum is lost has to be rebuilt then is lost again.
In Act Two, both classes performed it well. The teen agers were nearly flawless.
Maybe it was just nerves.