It's very unlikely that school would ever be cancelled because of rain.
That is unless the rain was causing flooding of roads where the buses go down to get the students to school.
So if the roads are flooding due to heavy rainfall then the schools will likely be cancelled but if the roads are passable and it's just raining then school would still go on.
School Buses have windshield wipers to allow the bus drivers to see so they would still run the buses and operate the school during rain.
You can't do much else anyway at home when it's raining so you might as well get the school work done so you don't have to make up those school days again.
If you have school cancelled then it takes longer to end the school year.
I've always had school when I was a kid even during heavy rains unless it was flooding the roads.