If you would like to stand on principle, write a note excusing your child based on the principle that the unspoken/unwritten agreement, based on precedent, is that school takes place M-F. Make your principle known politely.
Otherwise, to be excused according to BOE simply write a note indicating that your child didn't feel well. It should be excused w/ no questions.
There it is, principle or white lie.
The one thing a parent doesn’t want to do is that loud abusive Jerry Springer/Maury Povitch cliché “hell no, oh no you dint” at the top of your lungs which is only good for indicating your lower class upbringing.
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