I haven't the slightest idea, but I'm guessing it's a fairly large percentage, but, again, that's just a guess based wholly on my experience.  Everyone I know (knew) works (worked).  My parents and their friends all worked.  Most financial aide does not allow for living expenses, and, even though most students come from middle-class backgrounds, they still can't afford to not work.  I managed to avoid working for the last two years of my degree, but only because I saved while I worked full-time during community college (first year and a half), which costs only a fraction of what university education costs.

Be nice to America. Or we'll bring democracy to your country.