Here are the main problems with the idea of a raid training school:

1. If you do it on outdated content, it's worthless.
1a. Even a fresh 80 wearing greens will be ludicrously overgeared for any Outland content, which will trivialize large portions of each encounter. There won't be any challenge, there won't be any sense of danger, there won't be any of the excitement or thrill that makes a real raid fun.
1b. None of the gear will be upgrades, which makes people feel like they're wasting their time. DEing or vendoring stuff is nice in theory, but bear in mind that Outland enchanting mats can only be used for Outland-level enchants, and other than the still-rare Mongoose, there isn't a single Outland enchant worth getting these days.

2. If you do it on moderately outdated content (ie, Naxx), it's worthless.
2a. Again, most of the gear will not be an upgrade for people, which is disheartening.
2b. People will still outgear the encounters, which will allow people to get away with making mistakes that *should* wipe them. When my old warrior did Naxx for his new newbie guild, half our fights were just sloppy as hell, people were not listening to instructions, and when I tried to bring it up as a problem, I was brushed off with "Well at least we got the boss down, that's good enough." Well, no, it's not. The only thing people learn from sloppy runs is that it's OK to be sloppy.

3. If you do it on reasonably current content, people need to gear themselves up first.

That last one is the clincher. A lot of people do not understand the importance of running heroics. They want to jump right into raiding, but they *can't*, because everyone else is geared way beyond entry-level stuff. Blizzard is going out of their way to make it easy for newer players to catch up, it's now possible to get ilvl 226+ stuff in every slot just from running heroics, but if people can't take the time to gear up, well, they shouldn't raid.

Considering the life stage of the game right now, the number of genuinely new players who want to learn how to raid, *and* who are willing to put in the time... well, there aren't many out there. At this point, in any given guild, pretty much all of the people who don't know how to raid either don't want to learn or can't be taught, and all the well-meaning advice in the world won't help them.

Now, I do recognize that there are a *few* genuine raid newbies out there, and I'm a big proponent of running heroics with them to gear them up, working with them one-on-one at the training dummies to go over rotations and gear choices, then integrating them into raids. But even they would benefit more from an afternoon of chain heroics than from being dragged through Kara.

(Hopefully this doesn't double post, I waited like five minutes to see if it showed up the first time).