What are the traffic laws for a school 'walking bus'. Am i legally required to stop even if they continuung to cross for a long time afyer the light goes green ?

To unpack this: (This is my opinion, and is not to be taken as legal advice, or any other sort of advice) 1) Whether you spent 20 minutes at the crossing, or 20 minutes in traffic for a variety of other reasons is unclear here, but I highly doubt you were sat at a crossing for 20 minutes, traffic makes everything seem to take longer. 2) A red light means regardless of circumstances you must stop, an amber light means you must stop unless it's unsafe to do so, and a green light means you **may** proceed, **if it is safe and legal to do so**. In plain English green means you *can* go if it's safe, not that you must. 3) The fact that you think that regardless of who's right or wrong it's ok to scare children by moving forwards in a 2 tonne metal box as they cross the street suggests to me you probably shouldn't be driving in the first place.