guardians' association; parents' association
Refers to the organization or group of parents/guardians at a school, similar to a PTA.
I took on a role as an officer of the parents' association.
parent-teacher meeting
Refers to the event or meeting where parents and teachers discuss school matters.
Please be sure to attend next week's parent-teacher meeting.
授業参観 is when parents watch children in class, whereas 保護者会 is a separate meeting between parents and teachers without classroom observation.
Compound of 保護者 (guardian) and 会 (meeting, association).