When I was in college the standard acceptable limit before receiving an F was 3 grammar/spelling errors per 1,000 words.
I don’t know when you were in college, but today’s students have access to grammar/spell checking assistance in the word processing programs. There should be no reason for very many errors.
Also, I don’t agree that improper usage (such as mixing up their, they’re, their) constitutes spelling errors; it seems to be more of an ignorance of the proper usage and/or laziness to figure out the correct usage.