We know that pressure = force/area. When the string of the school bag is thin, its area is small and as such the pressure exerted on the shoulders or the hands supporting the bag is large. lip this case, the force acting on the shoulders or the hand which support the bag is equal to the weight of the bag. In case, the strap is broad as is usually the case, the area on which the force (weight of the bag) acts is large and the pressure is reduced as the weight of the bag is now distributed over a larger area.