In addition to @WEBjuju's answer for addressing the parents in another room:
The terminology that refers to "parents of a student" is
학부모(學父母)
An alternative that can cover "guardians and (older) siblings* of a student" is
학부형(學父兄) * the word calls for "father/brother of a student" but sibling is implied.
학부모 is used more formally in a context of addressing the general group of parents in the school or a class.
- Formal Example: "Dear Parent (of student). Please note that tomorrow's test is cancelled"
학부모님께 알립니다. 내일 시험이 취소되었으니 참고 바랍니다.
In a casual context such as text messages, this would be more appropriate.
- (More) Casual Example: "Hello Joy's mother/father. Tomorrow's test is cancelled so it is ok for Joy to miss school.
안녕하세요 Joy 어머님/아버님. 내일 시험이 취소되었으니 Joy는 학교에 안와도 되요돼요.
Your question regarding -님:
The age is not a main factor. If you want to be polite and respectful, you can append -님 even if the parents were younger than you, for instance. It's not limited to teacher-parent relationships.
Source: http://korean.joins.com/news/article/article.asp?total_id=2513233&ctg=
RE: 학부모(學父母) 학부형(學父兄)
