This is how I'd do it:
Start off explaining why you are writing, maybe even in bold type at the top. "RE: Parking in 'No Stopping' zones in front of school grounds"
Body of letter: you see loads of people doing it; you are concerned for the safety of the children.
Offer suggestions: cars can be parked further down the street and parents should escort children to the crossing; perhaps the school/council can recruit a volunteer lollypop person to man the crossing, having the double benefit of shooing away the illegally stopped cars and ensuring kids cross over safely.
Sign off: briefly recap your argument - the stopping of vehicles in those areas is unsafe and illegal. You would like to see that law enforced either by the school or by the council.

While passion is good, politely worded arguments are taken more seriously IMO. Good luck.