I remember reading an epistolary YA book when I was a pre-teen (so in the late 80s/early 90s). It was about a group of kids who go on a field trip to a fort. While there (overnight trip) the adults all get shellfish poisoning from bad clams they have foraged. The kids then take over, and end up defending the fort from a biker gang.
Stump the Bookseller
New format, same book stumper solving.
The War At Fort Maggie, by Raymond Bradbury. Set in Nova Scotia, published in Toronto, 1981. Link on my name discussing it.
I think this is it! It doesn’t look like it was very popular, but I used to re-read it often.