Hey jon- sorry for the delayed response.
I think you should explain more about HABs to your school/insurance to prove that the liability risk is very low - we got permission by doing this our first time.
Explain that you'll be sure to run predictions days in advance and right before the flight and use the software to show that there is no chance of going over a populated area (you may have to fly from a more remote area). These predictions are based on real weather models and are very rarely completely off. Also explain to your school that you need FAA permission and they'll only say ok if they agree you won't hit anything/anyone.
Show how you'll design out the risk. If you use a larger parachute your payload will drift further but will come down much slower, reducing any chance of damaging things on the ground. You can also add extra padding on your payload to ensure it is soft when it lands.
Explain that hundreds of balloons are launched every day, especially by organizations like the national weather service, and there are no reported causes of serious injury (that I know of) due to a falliing balloon.
If it helps, feel free to pass along my contact info and have them reach out to me as an aerospace professional at spacex - if word from another organization may help, let me know and I can try to make that happen.
Hope this helps and let us know if there is anything we can do!
