YA - Child/ potential bully in the story has an unknown brain tumour that erupts at the end of the book. It was a school book early 2000s/ 2010

[Tahlia]

Might have been a motorbike on the cover

[Kris]

Letting Go of Lisa by Lurlene McDaniel?

Google Books preview - https://books.google.com/books?id=kPi...

Google search of Goodreads:
site:goodreads.com "young adult" "brain tumor" motorcycle
Then click Google's "Images" tab.