Ben Bertoli, a sixth grade teacher from Indiana who has built an MMO called ClassRealm that has students, teachers, and administrators singing his praises.
Kotaku reports that the game is basically an experience point system that rewards kids for being studious and helping their classmates. There's also an achievement system and classes and races that include wizards, zombies, ninjas, and vampires. Bertoli initially started with something approximating a Google document but has since moved on to full-blown graphics and an app that works on mobile devices as well as laptops and PCs.
Bertoli has partnered with a computer engineer, a graphic designer, and an illustrator to fully flesh out the project, and there's also a Kickstarter campaign in case you want to contribute. Bertoli and company are currently slaving away on ClassRealm in their spare time, but the rewards are well worth it. "If [kids are] struggling in a certain thing, but they are pushing hard because they want the XP, or an achievement, and then suddenly it clicks, seeing that happen is really rewarding. The student feels like, 'I was going for XP, but now I am happy that I learned something,'" Bertoli explains.