"Hurry girls!" urged David, who had just returned to the laboratory after manipulating the lights. "They'll catch us before we know it."
But the young fugitives were too late. Just then there was the sound of many feet running up the stairs from the other door.
"How about one of the gallery doors?" asked Grace.
"They are all locked," answered David. "There only remain the skylight trap-door and the roof. Do you think you could manage it if I helped you?"
"Of course; we could manage anything," protested the freshmen girls.
It was an easy matter to climb up the ladder, and clamber through the trap-door on to the roof.
"We're just in time," whispered David. "They have found the right key to the gallery door, and they'll be coming in both ways. Crawl carefully now, girls, for heaven's sake, and don't slip!"
The seven young people began slowly to draw themselves along the gymnasium roof on their hands and knees. Fortunately, it was not a very sloping roof, and their only danger lay in their movements being heard from below. Meanwhile the gymnasium had emptied itself, and parties of enraged sophomores were engaged in searching the adjoining class rooms and passages.
"Let's surround the building on the outside," cried one of the class leaders. "They can't escape, then, by any of the fire escapes, and we are sure to catch them!"
In a few moments, David peeping over the edge of the roof, saw figures stationed at every possible exit, waiting patiently.