The mermaids wriggled in their coral seats, listening to the deputy head speaking from the rocky stage. Pearl sneezed five times in a row and Marnie gave her a tissue. Her ears burned with the effort of not looking at Orla.

Ms. Mullet’s voice echoed around the Assembly Cave. Her shell glittered with tiny barnacles and her small eyes swivelled on their stalks. Marnie had a feeling that the old crab missed nothing. There was no sign of Lady Sealia, except in the portraits on the walls.

“. . . and please remember not to chase the reef sharks,” Ms. Mullet was saying. “They can be unpredictable, particularly at lunchtime . . . I’m pleased to say that the library is open again after this morning’s unfortunate snapper invasion. Thank you, Len.”

A brown-and-white-striped lionfish sitting to one side of the stage looked up from his book, lazily waved his frilled fins, and looked down again.

Pearl leaned in to Marnie. “Lionfish are really venomous,” she whispered. “I feel sorry for the snapper . . . ATCHOO! Can I have another tissue?”

A small grey fish flapped up to Ms. Mullet and whispered something.

“. . . and Monsieur Poisson asks that no one enter the sea cucumber patch as our sea cucumbers are breeding. They taste funny if they’ve been disturbed,” Ms. Mullet finished. She snapped her claws. “Now, as you are aware, yesterday’s tour of the school did not take place.”

“And we all know whose fault that was,” Marnie heard Orla whisper to Mabel Anemone a few seats away.

“I will lead the tour right after this assembly.” Ms. Mullet’s eyes swivelled in the direction of the first years. “Every first year is to gather outside the Assembly Cave and wait for me there. Thank you for your attention.”

Marnie waited until Orla had swept out of the Assembly Cave in front of her. She didn’t want to give the black-haired mermaid another chance to play a trick on her. But apart from a dirty look, Orla ignored Marnie completely.

“I think Orla put that snake in my bag yesterday,” Marnie told Pearl as they swam out of the cave for Ms. Mullet’s tour. “She wanted me to get into trouble.”

“Why?” asked Pearl.

Marnie shook her head. “I don’t know. But it’s the only theory I’ve got.”

“Attention please.”

Ms. Mullet was perched on a ledge above the first years, who stopped chattering at once.

“Welcome to Lady Sealia Foam’s Mermaid School,” said the deputy head. “The school was set up by Lady Sealia’s great-great-great grandmother, Queen Maretta, to prepare young mermaids for life in the Seven Seas.”

This made the first years chatter all over again. They had been listening to stories about Queen Maretta all their lives. Marnie thought about her mom’s story last night. It was amazing to think Queen Maretta was once alive.

“All our classrooms, laboratories, kitchens, stables and library are built into School Rock,” Ms. Mullet continued, “all the way from the surface of the lagoon to the bottom of the seabed. Follow me.”

As Ms. Mullet scuttled away, the first years followed with a swish of their tails.

“Where do the merboys go to school?” Dora Agua wanted to know.

“Atoll Academy, on the other side of the lagoon,” said Pearl. “Lady Sealia’s husband Lord Foam runs it.”

“How do you know that, flounder-face?” said Orla, making Mabel Anemone laugh.

“I just do,” Pearl said, tilting her chin. “And I don’t have a face like a flounder, actually, because flounders have eyes on top of their heads and I don’t.”

Pearl was brave, talking back to Orla like that. Braver than Marnie. But that’s not difficult, Marnie thought gloomily.

“Keep up, first years!” came Ms. Mullet’s voice.

The Music Department was one level down from Lady Sealia’s office and the Assembly Cave. Marnie’s thoughts drifted as Ms. Mullet took them past the practice rooms and classrooms where Marnie had spent most of yesterday sitting in the corridor. She was taken by surprise when Orla barged into her, causing Marnie to scrape her tail painfully on the wall.

“Clumsy,” said Orla, narrowing her eyes.

Marnie tried to be as brave as Pearl. “I’m not clumsy,” she squeaked. “You are.”

“I didn’t do anything,” said Orla, folding her arms.

“Yes you did,” said Marnie a little desperately.

“. . . our Seaharmonic Orchestra is always looking for members, particularly players of the rock tuba and razor-clam flute,” Ms. Mullet was saying. “If anyone is interested, please see Miss Tangle. On we go.”

Orla flipped her tail and shot away down the rocky corridor with Mabel Anemone. Marnie swam slowly after the others.

“What did Orla say?” Pearl asked as they swam past the Oceanography Cave with its large observation window.

“Nothing,” Marnie muttered.

“Orla’s mean, isn’t she?” said Dora.

“Especially to Marnie,” Lupita agreed.

“Everyone’s noticed.”

“I reckon she’s jealous that Marnie’s related to Christabel,” said Pearl.

Marnie hadn’t thought of that.

Dora and Lupita agreed, and then started discussing their favorite Christabel Blue songs.

“I like ‘Wave Goodbye.’”

“‘Clamshell Heart’ is better.”

“She’s so funny on Radio SeaWave,” said Dora, giggling.

“And she has the most amazing clothes in those fashion shoots she does for Fishtales Monthly,” sighed Lupita. “Marnie, can we meet her one day? Please?”

“Sure,” said Marnie absently. She was still thinking about what Pearl had said. Could jealousy really be the reason Orla was so mean to her? It seemed very strange.

Like the Oceanography Cave, the Art Studio had large windows to let in as much light as possible. In the library, the books in their dark blue mussel-shell cases were lined up on rocky shelves as neatly as the stripes on Len the lionfish librarian’s fins.

“You can borrow as many books as you like,” Ms. Mullet told them. “Just be sure to put them back where you found them, or Len will have something to say.”

“I think it’s disgusting, having a poisonous librarian,” Mabel Anemone said as they swam on down the rock. “I’m never going in the library.”

“Lionfish are venomous,” said Pearl. “Not poisonous.” Mabel rolled her eyes at Orla. “Like that even matters?”

“You can eat venomous fish,” said Pearl. “You can’t eat poisonous ones.”

“Good to know for when we eat the librarian,” said Orla sarcastically. The deeper they swam, the darker it became. They passed the Sports Caves with their fishball nets and weightlifting equipment. (“The pressure of the water at this level ensures a good workout.”) The Dance Studio had a barre made out of a wrecked ship’s mast and polished crystal mirrors lining the walls.

“And now the stables,” Ms. Mullet said, pointing at a series of craggy oyster-shell half-doors near the bottom of School Rock.

Marnie drew back. She had never liked seahorses, with their bony heads and sharp teeth. How Aunt Christabel had been brave enough to steal one was beyond her.

Orla noticed her expression. “Is Marnie Blue scared?” she said with interest. She swam closer to one of the stable doors, holding out her hand. “Come on, little seahorse,” she said. “Show us your fangs.”

“Come away from there please,” Ms. Mullet said sharply. “Seahorses bite. The stable master, Mr. Splendid, is away this morning, and has asked that no one enters the stables until he returns.” She snapped her claws to keep the first years’ attention. “Now, the last place to see is on the seabed itself. The hot-water vents are used by our laboratories, Domestic Science Department and school kitchens. No dawdling please.”

“I dare you to go inside a stable,” Orla whispered in Marnie’s ear as Ms. Mullet swam off around the corner.

“I don’t want to,” said Marnie.

“Why? Is the famous Marnie Blue scared?” sneered Orla. Beside her, Mabel snickered.

Marnie’s heart bounced around her chest like a frightened prawn. She gulped, remembering Aunt Christabel’s advice: Don’t let them see that you’re scared.

“OK. I’ll do it if you will,” she found herself saying.