Hello
@Chelseaxo,
Apologies for the delay in answering. If you still need help, you can find platform 9 and 3/4 between the platforms 9 and 10. There will be a trolley, behind which you can run through a wall and enter. There's also various NPCs/students around that can give you hints if you click on them.
I hope this helps! Feel free to reply again if you have any other questions.
Regards,
Aurora