School's Out Forever

Just tried to two man this boss and he does not cast [spell=113395] more than once on the same target so you will need a group of five people to get this achievement.

I can confirm that you still need a full party to be able to complete this achievement as of 6.0.3.

Was quite simple to do this achiev. Our setup was:

Prot Warrior
Affliction Warlock
Elemental Shaman
Fire Mage
Holy Paladin

The Prot Warrior tanked the boss in the middle of the room where he spawns. The Holy Paladin healed with RF on so all the adds automaticly jumped on him. To avoid any dmg on them, he kited the adds on the upper part while the shaman would ocasionally throw heals on the tank in case the healer would get overwhelmed. All the 3 dps stood about halfway on the stairs and dotted / chain lighting the Failed Students to avoid them going on the healer. Just AoE those adds. Warlock and Mage stopped the dps at around 20% to let the healer get all the adds.
Once we had the 50 adds, the paladin just dispelled Explosive Pain.

A tip for warlocks: if you place your teleport at the bottom of the stairs, no matter which room you get teleported to, you will always have your teleport in range and can immediatly teleport out of the room saving you the hassle of dealing with any adds at all.

Edit: I guess the "downrate trigger happy" gang passed by.