You either mean the scroll of resurrection or recruit a friend.
For the scroll visit:
http://eu.blizzard.com/support/artic...ategoryId=2240
For recruit a friend:
http://eu.blizzard.com/support/artic...ategoryId=2240
Though since you have an existing account and you've played TBC, both options aren't really important to you (unless they changed the TBC requirement thing for the resurrection scroll).
The new expension pack changed A LOT. There's a new continent, new level cap. It pretty much changed WoW the same way the second expansion pack changed classic WoW (well, almost). Some say it's better, some say it's worse. You'll have to find out for yourself. :P
You don't need Wrath of the Lich king to continue to play, only if you want to access it's content (the new continent, leveling to 80, etc).
As for combining WoW with doing A-levels, it depends on how smart you are. :P But WoW requires at least 1-3 hours a day, depending on how good you are at playing, what you want to do and some things just come down to luck. :P