This is a wild hack. Go to translate.google.com . Set "Translate from" English to English. Paste the URL of a unblocked Minecraft provider (like an Eaglercraft server list). Click the translated link. Google acts as a proxy, fetching the game for you. The school firewall thinks you are just doing homework on Google.
For a more modern experience (like versions 1.5.2 or 1.8.8) that includes survival and multiplayer, community projects use Javascript/HTML5 to bypass school filters. Play Minecraft for Free Online play minecraft free unblocked