The LED and push-button switch are clearly a custom wiring job, no one else's bus will have the same wiring,
although the "batteries" switch may be a replacement for a burned out one, as the labeling style matches the other switches.
Since the "batteries" switch will let you start the generator, but only after 30 seconds, it may activate a solenoid that joins a weak battery bank to a strong one. Don't leave the switch on when the bus is parked, or the strong batteries will run down trying to charge the bad ones. You should hear the solenoid click somewhere down under when you operate the switch.
It's also possible that it controls a battery on/off solenoid, to disconnect the batteries when the bus is parked. If this were the case, the generator should not need 30 seconds before it starts. I have worked on some ambulances that use a switch and solenoid to turn the batteries on, as opposed to the more common use of various types of yachtsman's rotary switches for battery disconnects.
As far as the push-button and LED, you will have to either follow the wiring, or find someone who either wired or drove the bus to know what it was intended for. If there is some lettering or documents identifying which district used it, you might try looking up the number for the bus garage and asking the mechanics. If it was a nationwide operator like Laidlaw or First Student, heaven help you.
While a momentary function like windshield washers should be obvious, it may control something less obvious like an added ether shot control for very cold starting. I've seen these as a standard option on older snow plows, maybe a mechanic added one to your engine?