Municipal buses are parked in depots overnight, right? That would kind of make sense - they already have to own the land for that, the vehicles already have to stay there in specific conditions, etc.
I don't think that makes sense given that you'd probably want a full battery in the morning to start your day, and night-time is when you'd want to draw from the battery (assuming solar becomes a much bigger component of grid power).
It's possible I suppose that cities and school districts could have busses that have more capacity than they strictly need, and so it makes sense to use any surplus capacity for grid storage (while maintaining a reasonable margin in case a bus has to make an unexpected trip because some other bus had a flat tire or something).