It’s also expensive and heavy. If you want to stop rounds from an assault rifle, you’ll need rank B4 material:
Because the glass is so thick, the weight is increased dramatically. Regular windshield glass weighs around 25 pounds. A rank B4 windshield (generally around 20+mm thick) weighs between 150 – 250 pounds depending on the design…
Also, the windows are only a small portion of the sides of the bus. You’d also need to up-armor the sheet-metal body to maybe 3/8" steel. Instead of a 25,000 pound school bus (meat filling not included), you’d end up with a 75,000 pound school bus and a gaping hole in your school budget.
The school bus’s big advantage is mobility: if you’re a moving target, you don’t really need fortifying the way a stationary target does.