I'd rather get 500-1000 words an hour and go back to edit, than get 250 perfect words that I'm likely going to have to go back and edit anyways. The standard you talked about is related to the final draft, and the product of writing is rarely the product of the first unedited draft. Quantity has a very large value in learning to write.
I think in university the issue becomes largely one of limited time and effort budget. I was an engineering major and took some economics courses for a minor (which I never got in the end) that involved essay writing. The issue wasn't that I didnt know i could improve it by editing, it's that each piece of homework competes against each other piece of homework for time and attention. The goal is 'how good can I make it in the time I can justify putting towards it'.
I'm sure I made some boneheaded errors in some of my work but I was already doing 'homework triage' and leaving the least valuable but time consuming assignments to when I might be able to get to them.
Writing a work you intend to publish rather than submit to a teacher is a different exercise. It's hard to practice in an environment where you're already racing the clock to get your 4-6 hours of homework out of the way each night.