Probably your schools proxy is not allowing you to access the site. They have probably set it up like this for a reason, or there proxy was playing up at the time you tried to get on the website. If you can't access it next time you try , then most probably blocked - meaning unless you want to get into trouble by trying to unblock it , then just don't go on this site in school.
Yeah, It's more than likely blocked for a reason since schools use Proxy's to filter different sites. This is probably categorized under something they don't want accessed so they blocked it.