FLASH
The school instantly changed around Aaron. The door to the gym sealed off before a large, thick viewing window took its place to give outsiders a clear view into what was now a gym-sized fish tank. All around the campus, the classrooms and other places the students had normal access to likewise became tanks and pools attached by a network of tunnels and tubes, turning the school into a huge interconnected aquarium.
Some places remained the same, however. This included the hallways, which only became wider to accommodate large groups of outside visitors. Science labs and other specialized human facilities also appeared. Along the walls, signs and displays featuring educational material for the public appeared, much of it focusing on the discovery of mermaids as well as the history of the special school built to educate and study them while doubling as a public aquarium.
Inside the tanks, water replaced air, submerging all within. Teachers and faculty gained scuba gear and wetsuits, allowing them to breathe and get around underwater. Their bodies also became more athletic and healthier as befits active swimmers. They also gained the degrees and knowledge necessary to be called research scientists as well as educators.
Meanwhile, the gathered students underwent a more drastic change. All clothing below the waist vanished, exposing everyone for a moment before their legs fused together and reshaped into sinuous mer-tails complete with vibrant scales and brightly colored fins.
The girls became prettier, gaining figures and faces fit for beings of myth and legend. Hair grew down to their backsides as the color adjusted to many different hues beyond normal human coloration: blues, greens, pinks, metallic, etc. Some even sported patterns like stripes and spots! Finally, gills opened up along their narrow ribcages while webbing grew in between their long fingers.
At the same time, the boys all became girls, their manhoods fading away into female slits hidden beneath their scales while breasts pushed out from their chests. After that, they gained all the same changes as the original girls, joining them in lovely mermaidhood. Finally, everyone's upper body clothing shrank into matching uniform bras displaying the logo of the school/aquarium.
When the changes finished, Jennifer Tanaka the mermaid continued her speech in front of her peers, her siren's voice echoing through the water with musical clarity, albeit without any dangerous hypnotic elements.
"So, with all that said, don't forget to give our visitors a big smile when they come in. Remember, we owe a lot to the humans. They discovered our eggs frozen in that cave all those years ago. They revived us, hatched us, loved us, and raised us to be the proud mermaids we are today. Let's show them how grateful we are, and be the best mermaids we can be."
Outside in the hall, Aaron was now: