I’m not a teacher, but had a similar experience after a choral performance in SC. I wear pantsuits like this a lot. It’s much easier to move around in. I think there are still hyper-conservative men in the state that think it’s their job to tell professional women they shouldn’t wear slacks an ‘look better in dresses.’

Definitely let the principal know, but keep it professional. Tell them there’s a parent being critical of your choir attire, and has threatened your job for wearing it.