It's definitely possible, but not commonplace I had several friends rush as juniors and all got in. I actually heard seniors getting bids but for those seniors they knew like every single person in the sorority.

My college was in the middle ground of competitiveness, so it makes sense to me. Although many pnms were freshmen, girls from all class levels (undergrad only) rushed. It was not looked down, and definitely not discouraged to rush by your year although they provided a caveat to those upperclassmen, "it's definitely worth trying for but keep in mind the sorority is looking for all different things"

I rushed as a Senior though my case was a little different because I was a founder. However, I was involved in many activities on campus prior to being Greek!