What no one has mentioned is you have two different tenses in your story. Present and past tense. Your first sentence is present tense, then in the second sentence when you say the man in the swim trunks “noticed” her you changed it to past tense. Same with the sentence :”….being surrounded by Slytherins.” That part is present tense, the first part of the sentence is past tense. “Seeing she is nervous, the man provoked her.” Same thing, mixed tenses.

You want to write your scene in an active voice not a passive one meaning make the action happening now not starting to happen.

Like this:

“A crowd formed around the tank. Among them was a girl wearing shabby, dark purple robes. A man with midnight blue swimming trunks noticed her and said, “Ah, as you can see, my fellow Slytherins – or perhaps you smelled it already – a mudblood dares to challenge me.” The crowd turned to look at the girl. That was the first time the girl noticed she was being surrounded by Slytherins. “Oh my goodness”, she said to herself, “I’m in big trouble.” The man saw that she was nervous so he provoked her. “The sorting had did a great job. Mudbloods like you belong in Hufflepuff; the worst house at Hogwarts! Hahahahaha!”

Her temper rising she shouted, “Oh yeah?! Give me that quaffle and I will show you that everyone is the same!” The Slytherins snickered and passed her the quaffle. Holding it, her confidence evaporated; she had never played Quidditch before, or baseball either. In fact, this was the first time she had ever held a quaffle. While she tried to decide what she was going to do, the Slytherins around her laughed as one of them said “I’ll bet ten Galleons she won’t hit the target.”

“Well, I’ll bet fifty she won’t even throw the quaffle”, said another.

“Don’t listen to them”, she said to herself. “Think of a plan. Think!”

Just then she noticed Crabbe a few feet away diagonally to her left and Goyle a little further diagonally to her right. Suddenly she had an idea. Calculating quickly the distance and force she would have to use, she needed to get Goyle to take one step forward. Thnking of an insult, she said, “Oi, Goyle! Is it just me or have you gotten fatter than Crabbe?”

Angry, he started toward her. “Now!” she shouted, throwing the quaffle at the same time. The quaffle flew directly at Crabbe, causing the crowd to start laughing, but the quaffle, having a mind of its own, first bounced off Crabbe’s huge stomach then off Goyle’s gving it added momentum as it flew straight at the intended target making the man in the midnight blue trunks fall into the water.

“Yes!!” the girl shouted as she hopped with glee around the Slytherin crowd noting with satisfaction the surprise written on all their faces.

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A crowd around the tank is beginning to form among them is a girl wearing a dark purple shabby robes. The man with a pair of midnight-blue swimming trunks noticed her and said, “Ah, as you can see, my fellow Slytherins—or perhaps you smelled it already; a mud-blood dares to challenge me.” The crowd turned to look at the girl. For the first time, the girl noticed she was surrounded by Slytherins. “Oh my goodness,” she told herself, ” I’m in big trouble.” The man noticed how nervous the girl causing him to provoke her even more saying “the sorting hat made a great job. Mud-bloods like you belong to hufflepuff, the worst house in Hogwarts! Hahahahaha!” Instead of showing how nervous she was, she took the courage to fight it back that she shouted, “Oh yeah?! give me that quaffle and I will show you how everyone is the same!” The Slytherins snickered and passed her the quaffle. Holding the quaffle, her confidence began to evaporate. She never played Quidditch before, or baseball. In fact, this is the first time she got hold of a quaffle. While thinking what to do, the Slytherins around her laughed and said, “I’ll bet 10 galleons she won’t hit the target.” “Well I’ll bet 50, she won’t even throw the quaffle,” said another. “Don’t listen to them,” she said to herself. “Think of a plan, think!” Just then, she noticed Crabbe a few feet away (diagonally left) and Goyle, opposite Crabbe (diagonally right), but a little bit farther. An idea came up her mind. “Calculating the distance and force I have to use, all I have to do is make Goyle move one step forward.” Thinking of an insult, she said, “Oi, Goyle! Is it just me, or have you gone fatter that Crabbe?” Angry, he started at her. “Now!” she shouted, throwing the quaffle at the same time. The quaffle went directly to Crabbe. Realizing this, the crowd began to laugh. But the quaffle is not done yet. It bounced on Crabbe’s huge stomach and went to Goyle, which bounced on his stomach too. Then it went straight to the target, making the man with midnight-blue trunks fall into the water. “Yes!” the girl hopped around with glee, around the slytherins, around the crowd with surprised faces.

A crowd around the tank is beginning to form. Among them is a girl wearing shabby robes of dark purple. The man with a pair of midnight-blue swimming trunks noticed her and said, “Ah, as you can see, my fellow Slytherins—or perhaps you smelled it already; a mudblood dares to challenge me.” The crowd turned to look at the girl. That was the first time the girl noticed that she is being surrounded by Slytherins. “Oh my goodness,” she said to herself, “I’m in big trouble.” Seeing that she’s nervous, the man provoked her. “The sorting hat made a great job. Mudbloods like you belong to Hufflepuff, the worst house in Hogwarts! Hahahahaha” Her temper starting to rise she shouted, “Oh yeah?! Give me that quaffle and I will show you how everyone is the same!” The Slytherins snickered and passed her the quaffle. Holding the quaffle, her confidence began to evaporate. She had never played Quidditch before, or baseball. In fact, this is the first time she got hold of a quaffle. While thinking of what to do, the Slytherins around her laughed and said, “I’ll bet 10 galleons she won’t hit the target.” “Well I’ll bet 50, she won’t even throw the quaffle,” said another. “Don’t listen to them,” she said to herself. “Think of a plan, think!” Just then, she noticed Crabbe a few feet away (diagonally left) and Goyle, opposite Crabbe (diagonally right), but a little bit farther. An idea came up her mind. “Calculating the distance and force I have to use, all I have to do is make Goyle move one step forward.” Thinking of an insult, she said, “Oi, Goyle! Is it just me, or have you gotten fatter that Crabbe?” Angry, he started at her. “Now!” she shouted, throwing the quaffle at the same time. The quaffle went directly to Crabbe. Realizing this, the crowd began to laugh. But the quaffle is not done yet. It bounced on Crabbe’s huge stomach and went to Goyle, which bounced on his stomach too. Then it went straight to the target, making the man with midnight-blue trunks fall into the water. “Yes!” the girl hopped around with glee, around the Slytherins & the crowd with surprised faces.

I have to say, this was fun to read. i am a harry potter fan xD