Has anyone challenged "by the bed" student apartments as unfair housing?

But you're asking about privately owned buildings, not those owned by the educational organization, correct?

Some of it will depend on the local laws. I have been told that in my jurisdiction it is acceptable to limit new leases on zero-bedroom units (efficiencies/studios) to one resident. In effect, that means kids under 18 aren't allowed (since they can't sign a lease, and only one person is allowed in the unit). There may be a similar dodge in your jurisdiction. Realistically, if you want to limit the number of these buildings in your area, I'd recommend getting involved earlier in the process -- track what's being planned for development, who's buying properties and applying for zoning variances. Attend zoning meetings. I think a neighborhood has a much better chance of blocking future buildings from being built than they do of retroactively getting a type of building leasing scheme declared invalid.

When I had a kid as an undergrad, my university's lawyers were very, very confident that it was acceptable to kick me out of student housing on those grounds. The housing office actually tried to work with us on a way to stay in student housing and was then advised that it was too much of a liability no matter what we signed. My understanding was that housing intended for registered students was under no obligation to accept tenants who were not registered students. So there may be different legal issues for different kinds of "student housing"--if it's provided by the school, for students of that school, they may be on sturdier ground than if they were just generally marketing to "students." That is to say: it's plausible that it looks like there's precedent to discriminate based on how university-run student housing works, but that it really doesn't apply.

One edge case to search for would be whether these residences have ever done anything about a resident becoming pregnant.

These are privately owned buildings, yes. They're not associated with the local university or community college, and they don't formally limit residents to students; they just heavily market toward students. These buildings are in an unincorporated part of Santa Barbara County, if that helps or is interesting. I'd also be curious to see any news articles (or similar) about people trying to block this kind of apartment complex from being established, if anyone can find any.

There may be overlap between these and SROs, single room occupancy buildings, which historically serve a very different purpose.

My building is marketed to students (but is "by unit"), and we have some new luxury apartments NEAR campus that are pre-furnished. My understanding for my area (Philly) is that they are marketed to students, and make it easier for students (i.e., no credit check), but they do not preclude other residents. My building in particular is a combination of students and others, there is at least one baby in our building.

Is there a local organization that offers free or low cost legal aid? I think you'd need to ask a lawyer to understand what's relevant locally.

Student-only private apartment complexes are something Madison, WI has been trying to get away from. This https://www.cityofmadison.com/neighborhoods/downtownPlan/pdf/CharacterDistricthandout.pdf hints at the problem under the very euphemistically-titled "Mifflin Character Area" section, and it's to the point that developers are http://www.isthmus.com/daily/article.php?article=43504 http://host.madison.com/news/local/writers/mike_ivey/boom-town-there-s-no-end-in-sight-to-new/article_43955c1e-2a40-5ed2-824b-c8a174313f5e.html from making any more, partially because they simply won't be approved. Meanwhile, someone I know who works as a leasing agent for one of the big by-the-bed towers has said that adults' applications are generally "lost" or otherwise delayed until they can find a nice student to rent to instead. This keeps the college kids' parents from complaining, and isn't hard to get away with given our abysmal vacancy rate and that adults (with or without kids) don't much like the price tags on those beds in the first place. As far as I know though nobody's ever actually sued; I'll report back if our Tenant Resource Center has more to say on the topic.