I was part time for years in a school that made the staff give and do up to the max. But we were never expected to attend anything on our day off. However if we missed parents' evening we'd have to ring all the parents up instead. It was easier to just go! (Not in your case, obvioulsy as you have other plans.) I did know a teacher who gave the kids a note saying "Unfortunately I am unable to attend parents' evening. If you would like me to contact you with regard to XXXXXX's progress in science, please write a phone number and best contact time (Mon - Wed, 3pm - 6pm)" I think this, putting the ball in their court, means you only hear from the ones who really want to talk to you.