I was in the same boat as a high-school Spanish teacher (actually my first post here was asking advice about the same thing!) I planned to work until Christmas break, which would have put me at 34 weeks 5 days. I laugh at myself now.
It's very true that teaching is way more physically demanding than many jobs, and carrying twins is also more physically demanding than a singleton pregnancy. I was exhausted and plagued by dizzy spells and tons of BH contractions by 20 weeks, even though, like you, everything else looked fine (was carrying mono/di boys). After one really bad night where I almost went into L&D for all the contractions, I asked my OB to write me out of work at 24 weeks and he was happy to do it. Even though I look back and think it would have been nice to have worked longer, I know that my body was giving me definite signals that I needed to slow down. I ended up being put on modified bedrest at 32 weeks, even after I was done working, but I carried my boys until 35 wks 4 days, healthy as can be, but I'm convinced they would have come earlier if I had kept pushing myself.
Long story short: listen to your body. Only you know what is too much, and with twin pregnancies (esp. w/ mono/dis) things can change really quickly. You don't want to force an emergency to finally get some rest.
So ya, that's my experience/two cents. Good luck!