If they try to press charges, the photo evidence of the workbook and a statement by the kid will be enough to have it slapped down. Not to mention that the ones bullying tried to invade with malicious intent, being sprayed by a hose is the least of the offender families problems.
They have the potential to make it public and drag the mayor's name through the mud, they can even utilise the TV station who already have the previous interview about bullying to corroborate.
That dumb mother better watch herself.