As noted, many SBs put "student" in their profile when they are not currently students, for various reasons. I wouldn't be surprised if it's grossly overstated. Also, it should be noted that D.C. is a very atypical area in many regards, one of them being that is is the largest relative net "importer" of college students. Given that background, a whopping 44% of all SBs in D.C. list the word "student" somewhere in their profile, including as their occupation. The absolute number was just under 1,800.
Looking at enrollment stats, Georgetown = 18,000; GW = 26,000; American University = 10,000; Howard = 10,000; UDC = 6,000; Catholic = 7,000. Ignoring a handful of smaller institutions, that totals to about 77,000. Of course, not all the students live in the District, but I'll assume so for simplicity. Assuming that half the students are male (probably closer to 45% in reality), that means there are 1,800 SBs out of about 38,000 students, or something like 4.5%.
I'm sure that there is a huge variation by school and area, but something in the large ballpark of 1-5% seems reasonable to me.