Oh noes, a kid brought a banana to school. Call the hazmat team!

https://en.wikipedia.org/wiki/Banana_equivalent_dose

Edit: The BER of an average piece of 25% U glass is about 1300 bananas.

If it were possible to extract and enrich K40 from say 100 million bananas, you could kill everyone in the vicinity of it with a LD.

40 lbs box = 120 bananas at $13. So say $0.10 each. For a minimum investment of $10 million USD, you could have enough radioactive K40 to do some damage.. buried in tons of regular potassium that is expensive to enrich. Instead, for $10k / g, one could buy 1 kg of K40... if they could prove it was for a legitimate purpose.