Marco over at JMTC posted some good places to power level someone. I abused the RAF program as hard as I could. Once of my key spots was SM. From lvl 20 to low 40s I would grind SM Armory and Cath for XP. Here are some important things to remember.
All 4 wings are on the same instance ID. For as short as each one is, you should do 2 wings before you reset. Even then you will run into the lockout timer. I joined a XP farming group in SM cath once. Prot pally would do 1-2 pulls and kill everything. We did it 5 times and they said they would be back in an hour. After talking with them, I realized they had no clue they could also go clear armory between resets.
Do 12 min resets. If you are resetting in less than 12 min, you will run into the instance lockout. You should be able to do both armory and cath in 12 min. If it takes you too long, consider skipping the bosses. The little bit of xp they give for that extra 1-2 min they add slows you down. You are doing raw xp per 12 min here. If you blow away that 12 min mark, do the hallway in the graveyard (not the courtyard). That first room has 12-15 mobs that are quick to get too.
Avoid the library. Do it for the gear and the key and leave it alone. The mobs reset easily and respawn quickly at the start. It is just extra trouble for you low level chars.
Remember that a lvl 20 has an insane agro range. If the low level can see a mob, he will probably agro it. In both cath and armory, you have to kill the first mob before the lvl 20 zones in. If not he will usually die before the loading screen is gone. Remember you can get xp from behind walls and at a long range. There are safe spots
The first safe spot in the armory is in that first room. Stand to either side of the doorway. I prefer the right corner with back on the wall. I run the main into the courtyard and grab the mob on the stairs and round up the whole room. I pull them too that doorway and kill them just far enough away that the low level cannot see the mobs.
The second is on the other side of the courtyard in the far right corner. Put your face in the corner of that room. take the main down stairs and round up all the mobs on the lower level. If you kill them all near the bottom of those first stairs you ran down, that low level will get all the xp from them.
The third and final spot is down stairs. Go to the set of stairs that goes up and left along that wall in the corner is where you want to stick him. Not the little corner next to the stairs, but the corner of the room to the left. Run your main up stairs and agro the rest of the instance and pull them back near the top of the stairs.
In Cath I run the main in first and drag the mobs in the all into the courtyard. The alt runs in behind and stands to the left of the courtyard about 10-20 yards. Looking at the mini map, have him stand near the corner of the courtyard. With the main, drag everything to that corner on the inside and kill them there. If you have to do it in 2 pulls, move the alt up near the waterfall but make sure you kill the pat. (That pat would kill me all the time)
After the courtyard in clear, park your alt in the left corner next to the cath doors. I put the face in the wall. Run in with the main and agro the boss. Pull the mobs to the middle of the walkway you ran down to agro the boss. Killing the mobs here will give your all xp even when he is standing outside the room.
I was using a 70 mage to do my power leveling. I had to do the armory in 3 pulls and cath in 3-4 pulls. With a level 80 you can much larger pulls. I knew some 70 that could 2 pull cath but dying just a few times slowed me down too much. I have 11 chars over lvl 60 from RAF and I never ever want to step foot in SM again.