Hey Jacie,
I agree that 5-minute mysteries every day for 9-weeks, year-round
could get stale. However, if you broadened it to short stories you
could include a wider variety of stories (both fiction and non-fiction
narratives). I like the idea of having them then try to mimic this
sort of writing, as it would give them practice being concise and
trying to pack a lot of information into a small amount of text. I
think they would have a lot of fun with it.
If you decide to go with the non-fiction angle as Pat suggested, there
are great science resources out there (Scholastic) with short articles
and lots of text features to explore. There is also the creative non-
fiction genre which could be a blast. John McPhee writes some more
advanced pieces, but they have great detail and some gross-out factor
for the adolescent boys in your life.
Good luck!
Jocelyn