When I started college at a small, fairly liberal college in, ahem, 1986, we were first years/first year students. We were hardly the first class called that. It has been around a very long time.

ETA: I don’t know how common it is yet, so the only thing may be that readers may no understand what exactly it means initially. First year students would be what I would use the first time, then you can short hand it to first years. I wouldn’t use first year, because it’s a plural, as is freshmen.