Create restricted (non-admin) accounts for the people whose online time you'll want to control. Keep the initial admin account to yourself, and set in Parental Controls in System Preferences Time Limits as you prefer.
You can also set it so that you can control the account from another computers.
Set your admin accounts to have password protection so that 1) that account will not be logged in automatically after reboot, and 2) when ever you are away of that or any computer, you will have screensaver on with password protection.
That way none of those users will be able to use your account that was conveniently logged in to make changes in their accounts.
I would limit the total time online for the users, for all their applications, not just WoW.
That way, if they need to do e.g. homework or real work, it will be their choice when to do it. If for instance you'll allow a very generous amount of online time from 7 am to 10 PM, they can figure out when to do which activity and which one is more important.
With maybe some extras such as for any A they will get more online tiime, and any C or less they'll get one hour less.
