Edit; that is if the oxygen/gas content is high enough in the room.
Yes, in order to create an explosion their would have to be an ideal mixture of gas and oxygen. Once this ideal mix is created the slightest ignition source such as a lighter, turning on a lightswitch, or even a static spark would cause the fuel to either ignite or explode.
I'm not a scientist either, but I plan on becoming a firefighter so I know quite a bit about emergency response.
The quantity of gas would determine what happened to the school. If it would have ignited right off the bat the structure would remain decently intact but most windows would be blown out. With a large quantity of gas the structure would either explode or completely burst into flames. No good consequences for everyone inside.
What I am having trouble understanding is how students remained in the school until the evacuation. With the quantity of gas you are speaking of that would be highly toxic to breathe.