I attempt to go on normally, but if its too bad I just go home and rest...
(one day I was feeling iffy, and it got really bad at the end of the day, so I skipped after-school study and went home and threw up blood for the evening)
I can suck up a typical illness if it's mild (i.e. Colds, Sore Throat). I usually stay home if I have strep throat or any kind of disease that involves severe pain and vomiting. When I'm at home I lay on the couch and watch T.V. After a while I might go on my laptop or work on homework (if I happen to get what I miss from school).
R&R is pretty much my motto for sick days. Just gotta get some medicine in me, then I'll try to fall asleep. Of course, I haven't been legit sick in 2 1/2 years. Damn immune system is too good.