It's been done at my school before.
I help out at the library and the librarian is also in charge of the computer network in the whole school. I told her about the Net Send thing and she told me that she used to have another assistant many years ago that was always playing with Net Send.
For those of you asking, I'm guessing net send will only work on networks that ask you for a username and password. A microsoft windows network computer. I tried it at home, where we only share files but don't have to log on to the network, and it didn't work. I'm guessing that it'll only work at networks like those in schools.