Got away with one as a kid.....
My brother would bring me home 7.62mm shell casings from the Army. My Dad worked for British Gas so we had umpteen boxes of matches. Me and a school mate figured we could make a miniature cannon if we scraped enough match heads off. This progressed to packing the shell case and sealing with Evostick. A few nuts and bolts were added for good measure. We'd wedge it in cracks in the bike shed brick walls and blow chunks out of them. The "fuse" was a strip of paper covered in glue and sprinkled with matches stuck in a small hole drilled in the side. Igniting involved lighting the fuse and standing back. One day the fuse burnt out but no bang so I suggested my mate hold the lit match and get the tip of the flame in the hole in the shell casing.....
When we figured the copious bleeding wasn't going to stop with bog roll wrapped round his shredded palm, the spaghetti looking stuff hanging out wasn't quite right and shock was setting in, we went to the deputy head and I said I'd pushed him against a desk which had a broken hinge.
My "punishment" was to WALK with him to A&E and wait whilst they stitched him up!
Good times...