Nadia Harris had joined the mock trial team for one reason: the club met in Ms. Delgado's room, which had the only working air conditioner on the second floor. For two months she had happily played a witness, a nervous bakery owner who had seen a car accident through her shop window. Witnesses only had to remember their story. They did not have to stand up and persuade anyone.

Then, on the Thursday before the regional competition, Gabriel Ochoa broke his wrist at soccer practice, and his mother decided he would not travel. Gabriel was supposed to give the closing argument.

"You know the case better than anyone except Gabriel," Ms. Delgado told her. "You've been sitting through every practice."

"Sitting," Nadia said. "Not talking."

"Well," said Ms. Delgado, "this is the deep end. I'm sorry we have to throw you in."

Nadia spent Friday night at the kitchen table with Gabriel's notes, which were written in handwriting so tiny that she had to zoom in with her phone's camera to read them. She practiced in front of her younger brother, who fell asleep, and then in front of the bathroom mirror until her mother knocked and asked who she was talking to.

On Saturday, the other team's closing argument came first. Their attorney was a tall senior who never looked at his notes and who walked back and forth in front of the jury as if he owned the room. When he sat down, Nadia's hands were cold. She looked at Gabriel's notes and could not read a single word.

So she turned them face down. She thought about the bakery owner, the witness she had played all fall, and about what that woman had actually seen: a blue car, a wet road, a driver looking down at his phone. Nadia did not walk back and forth. She stood still, rested her fingers on the edge of the table, and told the jury the story in order, slowly, the way she would explain it to her brother if he were awake.

Her team did not win the competition. They came in third. But on the bus home, Ms. Delgado sat down beside her and showed her a score sheet. One judge had written in the margin next to the closing argument: Calm, clear, and believable. Hard to argue with.

Nadia read it twice. Then she took out a pen and, under the judge's comment, wrote three things she would do differently next time.