Sorry, but we couldn't connect to our servers. (Minecraft Mac)?

When I try to login to Minecraft on my mac it sais, "Sorry, but we couldn't connect to our servers." I'm on my SCHOOL MAC, but I know it's not blocked because I saw people in class playing it a couple of days ago. I tried to re-install java but it changes something deep in the computer so I needed the school administrator password to install it.