@Parentof2014grad – my son (high school student) traveled with his cello last year and was able to buy a seat for his cello on Southwest. He had planned on doing so again earlier this year and we were informed Southwest had changed its policy and now all cellos need to be checked. We did not pursue it further and opted to drive.
We further corroborated the policy change here: https://www.cellocentral.com/can-i-fly-with-a-cello/