Best inside fog stop chemical

Ok, in high humidity situations you can have the inside of your windows fog up. And the best solution is air circulation and lowering of the humidity. But my wife has a unique situation, she is a school bus driver. Tons of kids exhaling moisture, wet clothes from standing at the bus stop and a door constantly opening to let them on and off. So let's leave mechanical means out of the discussion and just talk about chemical applications.
I have put normal Rain-X on the inside of a car windshield before. Biggest mistake ever. Instead of occasional fogging it was constantly fogging. On another car I (got smarter this time) used Rain-X Anti-fog on just the passenger side of the windshield. But I saw no difference in the amount of fogging or in the speed it went away after starting the defrosters compared to the untreated drivers side. I have also tried foaming shaving cream applied and wiped off that showed no difference. Anyone use anything else with any success?