Meanwhile, hidden from the teachers, a roaring trade in talismans, amulets, and other protective devices was sweeping the school.
{Meanwhile|In the meantime|At the same time}, hidden from the {teachers|professors|instructors}, a {roaring|booming|thriving} trade in {talismans, amulets, and other protective devices|amulets, talismans, and various protective items|protective devices, talismans, and various amulets} was {sweeping|spreading throughout|taking over} the school.