The spell had a different function to the Imperius curse, although it also influences the victim's thought processes. The Ministry hasn't given me much information, but they have promised to keep me updated if they find out more about the spell.

As for why they targeted students, I can only guess that the caster wanted to target a group that couldn't fight back. Throwing off an Imperius curse is difficult for adult wizards and witches, and even more so for teenagers.