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A formal circular requires date and a subject line as below.
Simply address them Mr and Mrs. If this is a generic circular for all parents, you can use "dear parents".
I am making up the English name for 學校管理委員會, please replace it with the official name.
April 1, 2012 Dear Mr and Mrs Qasim,
Parents Teachers Association (PTA) Meeting
Please be informed the upcoming meeting for Parents Teachers Association will be held on April 31, 2012, 4:00 pm in Room 406 (Conference Room). We will be discussing the details of the "School Management Committee Board" election for the next academic year.
Your participation is crucial, we are looking forward to seeing you at the meeting.
Sincerely yours,
XYZ
Chairman of PTA