Another feature of this letter that was wrong was that it said that the only excuse they would accept for non-attendance would be a GP letter.
It is NOT part of a GP’s job to write notes for pupils who cannot attend school. Even those in employment are expected to sign themselves off for the first week - if an employers wants confirmation that the employee is unwell in this period it must be done by occupational health, as it is not NHS work for a GP to do it.
There is clear guidance for schools about when a doctor’s note might be needed.
For more information, you could try http://www.ganfyd.org/index.php?title=Get_a_note_from_your_doctor