This story is a response to this prompt.
"Anyone noticing anything?" Professor Thotep asked.
The class was silent.
"I'll replay the video once more." Once it was finished, he reiterated: "Who noticed something unusual?"
There were a few chuckles from the audience. After playing it four more times, Professor Thotep was confident that most of the students knew what was going on. He asked one of the students in the front row what was odd about the video.
"The character in the second shot is not only a different person, he's wearing different clothes!"
"That's exactly correct, and the time that it took many of you to notice this demonstrates humans' selective attention. This particular kind of selective attention is referred to as change blind--"
Before Thotep could finish his sentence, officers wearing black masks and body armor with jet-packs flew through the windows on the East side of the room, and several armed with heavy shields and taser clubs burst through the front and back doors on the West side of the room.
The slender, cloaked figure at the front of the room released a cacophony which he had spent ten thousand years perfecting to sound like hundreds of men, women, and children being slowly crushed and burned to death. Intimidation tactics weren't going to work on the agency's elite officers, they knew exactly what the target of the mission was and they were prepared to handle it. However, there was one possibility that they hadn't considered.
The students had long anticipated that this would happen one day. The student body of the entire school had developed a tradition: if you find out that you have Nyarla Thotep as an instructor, you always bring something to class that could be used as a weapon.
The officers new that they had a formidable enemy before them, and were intensely focused on how to take Thotep down. So focused, in fact, that they didn't take note of the many students lifting baseball bats and tennis rackets. They didn't realize that it was strange for many college students to carry loose wires and convoluted apparati formed of hot-glue and plastic-wear in their backpacks, nor did they appear to care that they had already moved past a great number of students and that several of them had just thrown a rope across the main aisle.
The team of six officers who rushed in from the back door tripped without knowing why. Simultaneously, two students who had procured makeshift slingshots and fired at the heads of the officers with jet-packs, knocking them out. Seeing two of their allies fall convinced the remaining ten airborne officers that the target's psychic powers were stronger than the enchantments on their linen masks, so they began to charge the transforming beast. The third set of officers, who had gotten the closest to the target, realized what was going on, but it was too late because they had already passed enough students who were willing to knock them in the head with their equipment.
Meanwhile in the main aisle, A few of the wire-wielding students had predicted the use of taser-clubs and had decided to bring highly insulative gloves. They quickly decided to wrap the wire around each of the downed officers' limbs and clubs. The guards were only down for a few seconds though, and the students only had time to entangle four of the officers before their recovery pulled wire out of the students' hands. One of the officers realizes that the students had tripped them and, infuriated, began to swing at the one closest to him; right before the blow connected, the officer made the mistake of also turning on the taser. All four the tangled guards fell back onto the floor, spasming as their clubs short-circuited and current ran through their bodies. The other two officers didn't last long as they were already surrounded by heavy-metal-object-wielding students. As the remainder collapsed, the students nabbed their shields and, along with the students who had defended the front door, ran to surround Professor Thotep.
The most experienced officer was at the front of the airborne officers, and he was ready to subdue the target regardless of how many shield-wielding children he had to knock down. He wasn't surprised when a student wielding a multi-pronged plastic fork spear jumped off of a desk right in front of him because he didn't even have time to register it. The spear went right through his face, but the momentum he had gained caused his corpse to keep flying, breaking the student's weapon.
Nine flying officers were still swarming about and dodging projectiles. They were able to fire a few shots at the giant atrocity that stood where the instructor once had, but the monster didn't seem to care. The slingshotters ran out of ammo, but the wire-wielders had just had a marvelous idea. They carefully wrapped their wire around the triggers of the remaining taser clubs and threw them at the flying officers. The first one missed, but the second one connected with one of the jet-pack fuel tanks. The explosion that followed was beyond what anyone was expecting. The blast knocked everyone to the floor and sent the remaining airborne officers flying towards the walls, knocking them out.
As the students recovered, ears still ringing, their instructor released a sound reminiscent of a mixture of nails and flesh being forced through a chalkboard tube by small, successive explosions.
"Excuse me, I was merely clearing my throat," explained the pile of eyes, claws, tentacles, and teeth. "Had I anticipated that superb example of fatal selective attention that we just witnessed, I wouldn't have bothered showing the video that many times," it said in a voice that threatened the sanity of it's listeners. The horror began shrinking and retracting its many protuberances, eventually re-obtaining the form of a tall, emaciated, pale humanoid with an elongated skull and white eyes. "Class dismissed."