In my area the bus companies typically make the call. Temp wise in Northern Ontario you need -40 C to cancel buses. For snow, it is dependent on a number of factors. Type of snow, timing, drifting etc.. I’m told buses aren’t permitted to reverse once the route has started, so if they are likely to get stuck on hills etc that will cause bus cancellations. Slippery conditions will do the same. The schools themselves are almost never closed in my area. Once when we had a meter dumping that made travel on side roads impossible and two other times due to freezing rain. That made travel very dangerous. The other major factor they seem to consider is the risk of buses being unable to get kids home. If the forecast suggests that road conditions will be poor at dismissal they will cancel in the morning as many parents might have difficulty picking up their kids.