The hearing test for a CDL typically requires you to whisper different numbers and letters into the examiner's ear from a distance of 5 feet and hear them correctly. You may also be required to take an audiometer test, which measures your ability to hear sounds at different frequencies.
If you fail the hearing test, you may be able to get a waiver if you can demonstrate that your hearing loss does not interfere with your ability to drive a school bus safely. However, the specific requirements for a waiver vary from state to state, so it's important to check with your local Department of Motor Vehicles (DMV) or CDL licensing agency to find out the exact requirements.