Minecraft isn't technically an MMO, but with multiplayer servers, you can have up to 999 players connected simultaneously in the same space, which is ironically more than many MMOs. But I digress!
Massively Minecraft (no relation) is a community of Minecraft players dedicated to the education of children on a massive scale through the popular indie sandbox. Created by virtual world vet Jokaydia, Massively Minecraft is "a Guild based learning community for kids aged 4-16 who are interested in developing digital media skills, exploring their creativity and developing online social skills." Of course, in an environment like that, there's an application process to get onto the multiplayer server's whitelist, so safety is a priority.
You can learn more about how the Massively Minecraft community operates by visiting its website.
[Thanks Tateru for the tip!]