It's probably blocked. You need to find a working proxy, if your school's proxy isn't efficient. Last year, people kept finding proxy sites that would work, and then they'd get blocked, and they'd find more. Shredsauce wasn't blocked, though.
This year I think they updated it and Shredsauce is blocked along with EVERY proxy site.
What I'm saying is that you need to find some obscure proxy site that isn't popular.