From the Admin console's Groups interface you can easily create a group and add users. You can add up to 25 users at a time, so copying and pasting users from a spreadsheet is really easy.
You can also create groups via the web interface at groups.google.com. When viewed via this interface you can create topics and have discussions in a forum style format. You can also setup a new group to act as a collaborative inbox (a solution similar to a shared mailbox in Outlook/exchange).
There is a group called classroom_teachers@yourschool.com - please don't delete it!