Kingston these restrictions are provided for safety reasons to ensure clear sight lines for both motorists and children outside schools. They are identified by yellow zig-zag lines (see example) and large yellow signs (see examples) placed either in the centre of or at the ends of the restrictions.
The times and dates of the restrictions may vary between areas or even between two different schools in the same road.
Please check signs carefully before parking or stopping on the markings.
No vehicles, including vehicles showing a Disabled Person’s Blue Badge, are permitted to stop within the area of the markings during the times and dates the restriction is in force, even for picking up or setting down of passengers. Additionally, you must not load or unload within the restricted area during the time and period that the restriction is in operation.
If the school lies within a Controlled Parking Zone (CPZ) then the Keep Clear section will also be covered by a single yellow line restriction which prohibits parking within the School Keep Clear section during the CPZ controlled hours. The CPZ controlled hours will not necessarily be the same as the School Keep Clear restriction. If you are not sure of the CPZ hours of control you must check the CPZ entry signs, permit parking bay signs or signs at a Pay and Display machine which will show the times and days of control.
However according to this news article, Bristol council brought in new rules for yellow zig zags that were already there. I would presume therefore that councils decide which ones should be enforceable and which ones are merely cosmetic. I would assume that the difference lies with whether it has a time plate. As for whether only council traffic wardens or police officers (or both) can enforce these I have no idea.