Vanilla Forums is free, open source software that anyone can use to host an online message board community. As you might imagine message boards powered by Vanilla allow your users to create member profiles, give you administrative control, and the ability to create sub-forums. Vanilla Forums software can be downloaded and installed on your own server(s) or you can use one of the paid hosting packages offered by Vanilla.
Applications for Education
If you have someone in your school that can install Vanilla Forums on a server, Vanilla could be a good way to build a school message board. You could also explore the hosting packages offered by Vanilla to see if they suit your needs.
Other options for hosting message boards to use in your school include Google Groups, Google+ Communities (if everyone is over 13), or Buddy Press (open source and you have to host it).