What if you have submitted the monthly fee of your child on timely basis and you still got a notice from the school that your dues are pending? There might be many such issues when you feel the need to register a complain to your child's school. You can simply write an email to the principal of the school for this purpose.
You can start the opening paragraph of your email by introducing yourself to the principal. Tell the recipient who your are and how many of your children are studying at the school.
Next paragraph shall explain the purpose of your email. Explain the actual problem you are having and which has compelled to contact the principal directly. Don't just put forward your problem but also present a possible solution so that the recipient may act promptly.
Conclude your email by thanking the principal and requesting to resolve your issue on urgent basis.