In general, I'd suggest the following:

• Make it optional. New players, that aren't new to mudding in general, hate being forced to go through Newbie schools of any kind. So there must be the option to bypass it. If they miss some useful info by skipping it - it's their choice.
• Make it interesting - no mobs in cages, no empty rooms you have to go to just to learn how to type NORTH. Have some funny mobs to kill, some minor quests to perform. Make it look and work like any normal area, with the addition of the explanatory signs, that they can read or not read at choice.
• Make it rewarding. Put some good equipment in it that doesn't load anywhere else in the game. Put some qualified info in some of the signs that not all players necessarily know about.
• Make it pedagogic. Start with the easy stuff, proceed gradually to the more advanced. Try to remember how bewildered you were yourself as a newbie.
• Make it in steps. In my mud we have 4 different classes, each teaching different stuff. You can skip any of the classes, depending on whether you think you know all about those things or not.
• Make it useful. Use the academy to give info about special features in your mud. We have one of the classes devoted to the use of ranged weapons, transporters and vehicles, since those features aren't found in any mud. The same goes for things like climbable and descendable objects, special commands like LISTEN, SMELL, FEEL - any features you have that are non-stock.
• Make it challenging. We have put a Quest Academy on top of our school teaching some neat tricks that only few players know. Each quest you solve leads you on to the next, and teaches you things on the way. If you give up, there is a solution to what you should have done in each room.

Just some food for thought...