QED by Ellery Queen

Morrell splits, Miss Carpenter picks up the envelope and takes a look inside, and she hollers, ‘I been robbed.’”
    The thin boy with the red hair called out, “So what are we supposed to do, drop dead?” and winked at David Strager, who had already sat down. The big blond boy winked back.
    â€œAnd your name?” Ellery asked the redhead.
    â€œJoseph Buell,” the boy answered defiantly. He was the one who worked at Kaplan’s, the big cigar, candy, and stationery store on 89th Street. “Who wants their old seven bucks?”
    â€œSomebody not only wants it, Joey, somebody’s got it.”
    â€œAaa, for all we know she took it herself.” And this was the third of the trio, the sharp-faced dark boy. If Ellery was right, he was the one who delivered part-time for O’Donnel’s Dry Cleaning on Columbus Avenue.
    â€œAnd you are—?”
    â€œHoward Ruffo.”
    The Three Musketeers, rushing to one another’s support.
    â€œYou mean, Howard, you’re charging Miss Carpenter with having stolen the teachers’ money?” Ellery asked with a smile.
    The boy’s dark glance wavered. “I mean maybe she took it like by mistake. Mislaid it or somepin’.”
    â€œAs a matter of fact,” came Louise’s quiet voice, “when I saw the money wasn’t in the envelope, my first thought was exactly that, Mr. Queen. So I searched myself thoroughly.”
    â€œMay I see the envelope?”
    â€œThis isn’t the one I was keeping the seven dollars in”—she handed him the envelope—“though it looks the same. I have a box of them in my locker there. The lock hasn’t worked for ages. This one must have been stolen from my locker yesterday, or earlier this week.”
    â€œIt’s a blank envelope, Miss Carpenter. How do you know it isn’t the one that contained the money?”
    â€œBecause the original had a notation in ink on the flap— Gift Fund for Helen McDoud .” She looked about and glances fell in windrows. “So this theft was planned, Mr. Queen. Someone came to class this morning armed with this duplicate envelope, previously stolen and filled with worthless paper, prepared to make a quick exchange if the opportunity arose. And it did. The class was milling around while Mrs. Morrell and I chatted.”
    The paper in the substitute envelope consisted of a sheaf of rectangular strips cut to the size of dollar bills.
    â€œAt the time you placed Mrs. Morrell’s dollar among the others in the original envelope, was everybody here?”
    â€œYes. The door opened and closed only once after that—when Mrs. Morrell left. I was facing the door the whole time.”
    â€œCould Mrs. Morrell, as a practical joke, have made the switch?”
    â€œShe wasn’t anywhere near my desk after I laid the envelope on it.”
    â€œThen you’re right, Miss Carpenter. The theft was planned in advance by one of the boys or girls in this room, and the thief—and money—are both still here.”
    The tension was building beautifully. The boy must be in a sweat. He hadn’t expected his theft to be found out so soon, before he got a chance to sneak the money out of the room.
    â€œWhat time does the first period end, Miss Carpenter?”
    â€œAt 9:35.”
    Every head turned toward the clock on the wall.
    â€œAnd it’s only 8:56.” Ellery said cheerfully. “That gives us thirty-nine minutes—more than enough time. Unless the boy or girl who planned this crime wants to return the loot to Miss Carpenter here and now?”
    This time he stared directly from David to Howard to Joey. His stare said, I hate to do this, boys, but of course I’ll have to if you think you can get away with it .
    The Strager boy’s full lips were twisted. The skinny redhead, Joey Buell, stared back sullenly. Howard Ruffo’s pencil twirled faster.
    It’s one of