It is very easy to feel like you are always behind. There is always something to grade, something to plan, or some kind of paper work to fill out. You may become overwhelmed and feel like you have to work around the clock.
Feeling like you have to work around the clock may cause you to feel like you have to work through lunch. This is a bad idea.
Even if you have a mountain of work hanging over you, take a break. Eat lunch with somebody. Socialize. Allow yourself to get away from the work that seems like an emergency but really isn't. It will still be there when you get back.
It won't kill you if something does comes up that makes you have to work through lunch every now then. Making it a habit, however, can be a major cause of stress.