There's one more step to either getting your classroom set up initially, or for tweaking it to get it perfect. You need to set up your tag vocabulary or vocabularies.
To do this, first go to your classroom. From there, click the "tags" button in the 'author tools' bar.
You will now see a vocabulary with a name derived from your username and a number. Beside that, you will see "Edit vocabulary" and above you will see "add vocabulary."
Go to Edit vocabulary. From here, you can change the name of your Vocabulary so that it is the same as your classroom name, or something else more clever, if you wish.
If this is the first vocabulary in the classroom you'll see boxes for "blog" "exercise" and "wiki" already checked off, halfway down the page. If you are adding a second vocabulary you will need to select those boxes yourself.
Scroll down to where there are a number of empty check boxes near the bottom of the form. You want to click off the BOTTOM TWO check boxes. Checking the first box will allow you to easily add your own tags on the fly, and the bottom box allows your students to view the tags you create.
Save your vocabulary.