After that, the students each drew their wands and glared viciously at the demons that had been penned in. But by the time Allen took out his pocket watch and saw that class was almost over, only Hermione and Draco, who had practiced privately in advance, could use it with any real proficiency. The others with a bit of talent could at most fire one or two, while even more students merely managed to make the tips of their wands glow a little.
After all, the casting rules for this magic were rather different from ordinary Charms, and it was the young wizards' first time coming into contact with it; they had no past experience to draw on. Although Michael Corner successfully fired one missile, because he had been distracted before casting, he set his target on Seamus Finnigan, who had turned Magic Missile into something resembling the Thunderclap Explosion Curse. Fortunately, Allen countered it in time, or even with the practice wand suppressing the spell's power, Seamus would inevitably have spent half a day in the hospital wing.
Having witnessed the wizards' enthusiasm—and the accidents that could so easily happen beneath that enthusiasm—Allen, who felt the reality was a little more dangerous than he had expected, had no choice but to warn them, "Before you master it, don't use it on people in ordinary duels. Also..." Allen's expression became extremely serious. "Whether it's Magic Missile, or Protego and Shield Charm, which you'll be able to learn very soon, force-field spells of this kind can't cause permanent harm to ghosts, but they may temporarily make it difficult for them to maintain their forms... So these Charms are strictly forbidden against ghosts... Of course, Peeves is an exception."
Although Allen did not say it outright, the young wizards all knew this