As a frequent Minecraft player I can say that the game is.. well.. partly unrealistic, like as said floating things and hostile mobs and stuff, but the building bit of the game is amazing, people collaborate and build with lighting and more that few other simulation games spesifically designed for the task can provide.

It is a huge leap to jump into using Minecraft at school, but if it's used correctly it could be a great tool.

I've heard an example of a student using Minecraft as 3D modelling software for his school project, and he got quite a good grade for it - so it is possible.