Given that I miss school for training (Thursday and Tuesday mornings, 8-12:30), I definitely miss school for competitions. Usually it's a Friday, and sometimes a Thursday as well. Occasionally a Monday, but that's rare.
Usually I only miss five or six days of school a year because of an actual competition, and sometimes a few more days for travel. I missed a whole week once because of a comp. in Bulgaria. Depending on the length and importance of the competition, I may or may not get some extra time for assignments. At my school in Grade 11 and 12, homework isn't collected or checked (the theory being that we're old enough to monitor ourselves), so really I don't have to do it (although I do when I have time). If I'm missing a day where there's a test or something, I usually write it or hand in the paper or whatever a few days before I leave for competition. I go to a private school with a lot of athletes (we actually have a few Olympians who graduated from my school), so teachers are generally ok with it if I miss class or hand something in a little late. It annoys my friend, who thinks I get 'special treatment' from some of the teachers because of my sport, but honestly, I don't think I would get the extensions and such that I do if I wasn't as respectful and kind to the teachers as I am. OT, but I always think it's important to have a good relationship with your teachers, so that they're more inclined to be accomodating if I have a competition or training or whatnot. The only person who has a problem with my attendance is the principal, but she hates me because I back-talked her in Grade 8. And since most of my marks are in the high nineties and I'm taking two Grade 12 courses, she can't complain to my parents anymore.
I don't worry too much about missing school; as a competitive skater, I've learned to manage my time. It's really hilarious at my rink; while the ice is being resurfaced between competitive sessions, all of the school-aged skaters are in the lounge getting fifteen minutes of homework or studying done, while still in our skates. Pretty much whenever I'm not training, I'm doing schoolwork.
Edit: Just to clarify, I'm not saying that school isn't important. If my marks weren't as good as they are, I wouldn't be skipping school. But as long as I maintain my average (I'm currently on the Distinction Roll - a higher average than Honour Roll), I don't see the problem with missing school to train. It's all about being responsible and managing time.