It means just what it says. If you recall, it's an adult's voice, and it's a fade out on "Another Brick in the Wall, Part 2."
The whole song is about teachers abusing their students, and their students standing up to them. Back in the period where that part of the narrator's life is supposed to take place, it was very common for students to either live at boarding schools or to go to schools where lunch was served. It's just a teacher being abusive. He (the teacher's voice) follows it up with, "You, you behind the bikestands! Stand still, laddie!" Presumably, the teacher has caught someone doing something they aren't supposed to be doing, and he wants him to stand still so he can get over to him and punish him.
I really don't think there's any deep meaning to that phrase, other than, "Eat your nasty lunch or you can't have any desert." That's just the type of thing an abusive bully of a teacher would do--take away the good stuff because the student didn't eat the nasty stuff.
If you think about it, parents, too, do it all the time. Kids have to eat their regular meals in order to be able to have their treats for desert. However, there's a difference between a parent doing it and a nasty teacher doing it. Later on, toward the end of the album, it talks about how the teachers are nasty because their wives are nasty to them. It's just a comment on that whole English school culture.
I've listened to "The Wall" regularly since it came out, and pretty much know the lyrics by heart