I've actually wondered this myself.
I was in grade 12 of high school when I was preparing to go for my grade 10 piano exam. As the exam date grew closer, I was nowhere near ready to pass it, so I started dedicating about 4 hours of each day to piano practice. I stressed so much and lost a lot of sleep, and basically my grades slipped because of it. At the same time, though, I didn't really want to make a career out of music and so was applying for other programs at various universities and had to keep my grades up, especially in English, the math courses, and the sciences. Also at the same time was working a part-time job, being a school prefect, and being in the school band (playing the clarinet), because as my dad said, extra-curriculars were good to put on university applications!
Which got me wondering...how in the world does anyone do well in school AND in piano (or any other specific skill)? Unless you are some sort of super genius, is it possible to do both or would you have to choose? :S