Re: Ha!
Cool story, Bro. Shall we break it down?
A standard croquet ball is approximately 3 5/8 inches (92mm). This translates to (roughly[0]) a 6-pounder. According to my "field guide", the maximum powder charge for a 6 pound gun was about 1.5 pounds of powder. Loading one with 2.5 pounds changes it's designation ... it becomes what we call "a bomb".
The wooden ball being "just the right size" suggests that the operator had no concept of windage (the intentional gap between ball and barrel). This would further cause over-pressure, leading to bursting (the ball itself being turned to powder/toothpicks in the same instant).
This is to say nothing of the fact that an open cannon barrel automagically becomes a collector of all kinds of detritus, especially when stored outside. This, when combined with rain water and/or condensation becomes a soggy mess at the bottom of the barrel. I seriously doubt the touch-hole would be functional in such a scenario.
Also note that the build-up of crap at the bottom of the bore would put the powder charge above any banding or other re-enforcement surrounding the powder chamber, further making the explosion likely (if it wasn't too wet for the powder to burn at all ...).
I wont get into the concept of a schoolboy making 7.5 pounds of functional gunpowder ... it's a lot harder than knowing the ratio of the chemicals involved, placing them in a bowl and stirring. It has to be a very fine powder, and very well mixed, in order to go bang. Most people trying to make it in bulk manage to burn themselves rather badly before it gets to the boom stage. (Yes, I know, we've all made small quantities. Knowing how, would YOU try to make several pounds of the stuff in a school chem lab? That's what I thought.)
As a wrap, have no fear ... all the kids around here with access know that guns are tools. They would no more attempt to fire a cannon (or any other firearm) inappropriately than they would use a hammer as a screwdriver. Education, from an early age. Whodathunkit.
[0] As a former British Naval Officer, they were probably French pieces, and back then we still had the awkward Troy vs avoirdupois (and several other "standards") translation difficulties.