I really think the best way to study is in complete silence, but maybe it's just me. Anyway, I think that exploiting classical music for that purpose is so sad. Classical music deserves full attention of the listener to be appreciated. I personally would consider that behave very unrespectful toward the great art, since composers didn't think their music as a bunch of background sounds. Maybe, a more appropriate genre of music for that use is New Age, or just environmental music. But I suggest silence and will.
For me personally, it's instrumental music in general, not just classical. I sometimes listen to instrumental post-rock (Explosions in the Sky is a popular example) when I'm studying or doing homework and I've found that it helps a lot, but it might just be me. Some people don't get the same effect out of it.
For me, I love classical music. I love it, because it's my favorite genre of music, and I like to play it on the piano as well. The thing about classical music is the rhythm of the music with it's specific instruments, mostly the strings and the piano, with the combination of tempos, arpeggios, measures, and speed, is that it forms complex sounds that helps stimulate the brain as it listens to these different types of variations, thus helping to make you and the brain cooperate with the complexion of studying ;).