There is no clear law regarding the difference between 'standing' and 'parking'. The intent in erecting the sign would seem to dissuade parking (i.e. parking with no one in the car) while a 'no standing' sign would seem to indicate that you should not be there at all. Arguably the officer could say you were blockading traffic. The best thing to do is ask the police about the parking/standing policy is in front of the school when picking up your children.