Let me start by saying I don't know if a waiver is necessary or not.
I do know that when I was working with teenagers a half decade or so ago, I had to sit through a three hour class with a lawyer talking about the ins and outs of liability law, as it pertains to guardian adlitems in the state of florida...
She said there are three key things to make parents agree to in relation to taking their children on a trip.
Firstly that they "forever hold harmless" you, your spouse, your business, your friends, you and your spouses estate, the estates of your children, your associates, known and unknown....
Secondly she said that if something ever happened and you apologized, you were admitting to negligence, possibly even up to or including willful and wanton negligence. She said once that happens, you are screwed beyond hope.
Thirdly that they accept any liability (financial, or otherwise) for anything that might happen to their child while in your guardianship. IE if they break a leg, the parents will be bound to pay it, not you or your group.
I think if you incorporated those three aspects - forever holding harmless, getting them to admit that they are willfully and wantonly being negligent with their health, and that they accept any and all forms of liability that might come as a result, that you might be reasonably covered.
Also I've had to sign waivers saying that I know super hots are not fit for human consumption, and consuming them is at my own risk....
I'm no lawyer... I don't know law. I just know that the lawyer lady scared the fire out of me, and made me not want to chaperone anything. ever.
ALSO apparently its against the law in the state of FLorida to offer legal advice if you are not a lawyer... So don't take this post as such... Talk to an actual attorney about law, and to the health people about health stuff....
I'm just a pepper farmer