YA fantasy; boy goes to magical school, time passes quicker, he wants to escape

[Eva]

A boy gets lured to a magical school by one of the assistants of the owner of the school. Time passes more quickly in the school. There is a lake in the school, if you jump, you get turned into a poisonous fish. He figures out that the school is turning people into evil people. He tries to escape but the assistants of the school try to stop him. He finally escapes with his friend and he finds out that his parents have aged a lot. He goes back to the school and tricks the school and it destroys itself. Everyone is saved.
Grade levels 4-7, fantasy, YA

[Eva]

The Thief of Always by Clive Barker