Being on your feet all day makes you tired. I think your body is trying to tell you to slow down. If it's possible, you really might consider half days so you get some time to rest. You're going to need that rest once you're in labor. I wouldn't worry about going into labor at school though, unless you really intend to work right up until the last possible second. Having had the experience of having babies come early (36w, 3d and 35w, 5d) I wouldn't suggest anybody work right until their due date but some people really do feel fine the whole time. It doesn't sound like you really feel fine though so slowing down may be in order.
Woke up at 6 am in labor had ds 6.5 hours later.