Writing - does anyone else have this problem?

When writing something, like stuff for school that actually matters, you can know exactly what you are supposed to do and exactly what you want to say but nothing comes to you? I don't mean just writer's block, it's worse and I've always been like this. Say we'll be doing some type of timed writing in cl - I'll sit there watching everyone write pages and I usually can't manage to even write anything. It's not a matter of having to brainstorm and organize my thoughts - I have no thoughts to organize. I've tried looking at those free essays online (not to copy!) for ideas but I get nothing. Is anyone else like this and what do you do?