DFD
you could use the process (cirlce) for the "Posting" and "Registration/Login/Logout" and "Searching" etc, and similarly you would make sure to involve a database of all the user details, as well as a database where the entire forum's posts etc are stored. The external entities would pretty much be students/teachers.
Make sure you also include the specific data on the data flow lines between each thing. it wouldnt be too hard to make up the different kinds of data that could possibly travel and move between the differing processes, entities and datastores