I would not waste my money on the usual preassembled survival kit. You can easily do better by assembling the kit yourself. in fact, I just finished an REI sponsored session on the gear for emergency preparedness in which the instructor also had a negative view of those kits.

How many do you need? one for each kid or just a few for those who might be in transit together? This could be a good class project...

Are there any criteria for what is judged sufficient by the school board?

Any emergency equipment on the snowmobile or plane itself?