I know exactly what you mean. For me, it was in Algebra II/Trigonometry, which combined both subjects into one year. It was the most advanced mathematics class I could take at that level, but it went so incredibly slowly. I would be gone for weeks at a time and come back and STILL be ahead of the class. Same thing for chemistry. In fact, if it weren't for the assignments we have to do in all the classes, I probably could've gone to school once a month and still made very good grades (heck, I practically did that and got 'B's...it's a good thing the teachers liked me and saw that I was making effort, or else they could've flunked me purely on the grounds of missing more than 15 days from each class). Still, it was hard to make the arrangements to make up all those tests.
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"There are things you need not know of, though you live and die in vain,
There are souls more sick of pleasure than you are sick of pain"
--G. K. Chesterton, The Aristocrat