What Ben said, but more particularly, When you schedule your class you are prompted to give it a title and description there is another box called "keywords". That is where you would place the community tag.
The following is from a post by Sid (Sid Guru at WIZIQ):
For example, if you are having class on English language you will need to add "English" in the keywords. Similarly if your class will be benefiting the members of other communities like the e-Teaching Community, you can add keyword "e-Teaching". Once approved the class will be available under upcoming classes in that community.