Is the hyphenation in this sentence correct?
- Full-time students can't work full time and expect to get top grades.
Yes, it is.
When full-time is placed before the noun, it is a compound adjective and takes a hyphen. When placed after a verb, it becomes an adverbial phrase and no longer needs a hyphen. It's as if you wrote "Full-time students can't work all day and expect…." You wouldn't hyphenate "all day" in this case.
The same logic applies to part-time:
- Part-time students take courses part time.