How much does a startard bus weigh?

You would have to define what you meant by a 'standard' bus. There are small buses, there are commuter buses, there are school buses, there are tour buses, there are coach buses.

For a small bus of the sort mounted on a van chassis (such as the Ford E-series), you'd be looking in the range of 15,000 - 16,000 lbs. GVWR.

I've delivered school bus chassis to Thomas Built and Blue Bird bus companies, and many of these chassis had hydraulic brakes, which are typically indicative of a vehicle with a gross weight rating of 26,000 lbs or less.

A commuter bus typically has a single rear drive axle, for which the maximum allowed weight rating is 33,000 lbs (this may be increased to 35,000 for intrastate travel on secondary roads).

A coach bus will typically have a single rear drive axle, augmented with a single tread pusher axle, so you're looking at the 40,000 lb. weight rating range, give or take.

Keep in mind, these are loaded weights.