As most schools finish their day araound the same time it is impossible for a traffic warden/parking attendant to be at every school every day. Remember, the lines are there for the safety of yours and other people's children. If there is a continuous problem contact the local constabulary through the head teacher.
Most people who abuse the lines, and I speak from experience, will just give you a mouth full of abuse if you approach them to give them advice. Take their car number and pass it on to the police. JD