"Student" with approx 70 jumps (lots of solos!) request a two-way coaching jump.I agree (always a good opportunity!).DZ operates a Porter , but it was a busy day , so a Caravan was flown in to assist.First time for "student" to exit Caravan.I decided to door jam Caravan ( at this point static ...being refueled) with "student" (being floater for this twoway) in order to simplify exit on actual skydive.On the count "..ready ...set ...go", .........yep , he goes.Falls 4ft onto the runway and breaks his wrist.....! I still haven't heard the end of it(from my mates!)....and doubt if I ever will!