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There is an interesting website designed to help schools with setting up Minecraft accounts.
http://minecraftedu.com/
According to the MinecraftEdu team, if you can arrange for the institution to purchase the game copies, a deal with Mojang will be made so that the game can be used for educational purposes at a reduced price.
I think this is a great step forward. It's more accessible than Second Life, less laggy, less money-focussed. There's also a self titled "Minecraft Teacher" who is trying to use Minecraft in his lessons.
http://minecraftteacher.net/