A "crowded classroom" refers to a classroom that has too many students for the available space. It means there are not enough seats, desks, or room for everyone to comfortably fit and learn. Full definition
If this trend continues the only thing anybody will achieve will be more crowded classrooms with even fewer quality teachers. (c3s.nyc)
I also remind them that the environment they're in — a small, crowded classroom surrounded by lots of teenagers — is a very difficult place to fully relax. (edutopia.org)
He sent subjects dressed in t - shirts with large images of Barry Manilow on the fronts into crowded classrooms where students were filling out a survey. (lawyerist.com)