Discipline - in study (akin to all parts of musical practice) and in attendance (would I skip an opera rehearsal just because?)
Preparation - studying thoroughly (would I be unprepared for my recital jury?), putting in my share of the work (would I let my scene partner down by not knowing my part?), and caring enough to put forth my best effort ("C's get degrees" but would I want my director to think of me as just average?)
These go hand in hand with the production of quality work, which shows a level of self-respect, which in turn leads to gaining others' respect. Earning and maintaining that respect with employers/co-workers (aka director, stage manager, leads, chorus, costumers, make-up folks, set construction folks, etc.) is what helps us keep up a good image in the singing world, thus helping provide jobs. And news of those acting less than professionally travels fast in the small world of singing.
The way we act in school is a chance to practice these lessons which are essential to professional well-being! For me personally, these are hardly my reasons for working hard in all aspects of school - I really just like academia - but they are skills I recognize having built along my journey, both naturally and through some hard lessons learned.
There are many singing professionals who have written about this, too, and in greater depth. One of my favorites is Cindy Sadler in her Classical Singer column "Ask Erda." Look her up! She doles out the tough-but-best love.
*Of course I say this and my brain starts singing The Last 5 Years soundtrack... :)