Stuck in Neutral by Terry Trueman. Definitely. I probably read tons of sad books, but I remember this one as of right now.
And reading the posts before me:

I also agreed Night was a sad book... I read it last year in class - it hits an emotional nerve with the violin boy... that really sad music, I thought I was able to hear it.

Meh. I didn't like the Outsiders. Ponyboy annoyed me. I wanted him dead instead.

Both times I almost cried when I read and watched Of Mice and Men.

Supposedly The Red Pony was supposed to be all sad and stuff but I was just angry. That kid needs lessons in life bad. *mutters* stupid kid....* That's why I don't really like any of his book, even Of Mice and Men wasn't that good.