Perhaps appeal, contest, or dispute.
Appeal is commonly used in the context of challenging court decisions by taking them to a higher court, but it could be used in an academic setting as well.
Contest essentially means challenge, but doesn't have a specific institutional sense associated with it.
Dispute also means challenge, and is often used in the ...
Another term is simply conflict-avoidant . I think that 'pushover' has a more negative connotation, so if you were trying to describe this more neutrally I would use
or, similarly, conflict-averse
My co-worker is very conflict-avoidant, so it can be difficult to get his real opinion about the problems we are having on the project.