You know, like the ones on General Health and Employment which warn us that the MNers responding to threads haven't been checked for qualifications, etc., etc.
This one would go as follows. Ahem ,<clears throat>
Please be aware that INSET days are not a part of the school year, but are days taken from teachers' holidays. Your children are getting exactly the same amount of education time as they always had. Furthermore, your child's teacher will have had zip input into the timing of INSET.
With any luck, it will quell the perennial whinges on this topic.