The Mind Controlling Priest can break it early
The Mind Controlling Priest can break it early. You don't stretch out the MC and let it break on its own. Break your own MC after your student has served its purpose.
After your taunt and bone shield cycle is over, right click on the portrait of the mob and release pet.
The third tip is to communicate. This is the difference between good Priest and bad Priest. Bad Priests hardly say anything and expect their partner to intrinsically know. This may be okay in some guilds. But when starting out or when in a pickup group, it's helpful to start talking to the other Priest.
The fourth tip is to watch your cooldowns. Bone Shield is the one you should pay attention to. Assuming that you're not using any timer or cooldown addons of some sort, you can observe the progress of the cooldown. When you hit Bone Shield and the indicator is at 50%, cue your partner to taunt.
The fifth tip is to bring the students to the tank. We set up our tanks in such a way so that there is one on the left side of the room and one on the right side of the room. The students are divided the same way. When a student is done tanking, it is run over the nearest tank and dropped before snatching up a new one.
Note that you don't need hit gear to do the encounter if you're a Priest on Mind Control duty.
This doesn't mean that every guild has the luxury of having two raiding Shadow Priests. Chances are as a healer you'll have to get into the hot seat and do it yourself. One of the Shadow Priests took an extended break from vacation. This left us with three Priests: 1 Shadow, 1 Discipline and 1 Holy. The other Holy Priest did not want to do it. The followings are a few tips we want to share with new Priests who have no idea what expect.
The first tip is to disable your UI. At first, we had trouble with this and can't properly releast pets with the current UI configuration we had setup. We simply canned whatever unit frames and resorted to the stock UI in order to manage the MC process.
The second tip is to break your MC intentionally. We've observed Priests who simply let their Mind Controls expire. They try to stretch that Mind Control as long as possible before the students eventually break free. Who do they gun for? They head straight for the Priest. The reason why Priests get caught off guard is because they're not expecting the Mind Controls to break after 40 or so seconds.