Since you're on Windows 2008, the way to do this has changed a bit. Here's what you need to do. Open GPMC.MSC and select the folder with the computers you want this to apply to (or the default domain). Right click on it and select "Create a GPO in this domain and Link it here.." Give it a name, ignore the starter gpo thing.
Then, Right click the GPO you just created and click edit. Navigate to Computer Configuration\Policies\Windows Settings\Security Settings
You should see Restricted Groups listed there, Right Click it and select Add Group
You can enter anything into the box that comes up. This will bring up the box that lets you determine where to put the users.Click the Add button next to the Members of This Group and enter Steckle Teachers and hit okay. Then select the Add box next to "This group is a member of" and enter "Power Users"
Hit apply and the users you want will be added to the Power Users Group.