For the benefit of those not familiar with the Yak 52, it may be worth pointing out how that srewdriver might have got where it got. Anything dropped into the cockpit, either directly or by falling out of someone's pocket, can very easily end up in the position that screwdriver was found - jamming the elevator mechanism at the very rear of the fuselage.
And as the report says, a screwdriver is essential for pre-flighting a '52 to release the covers for the fuel and oil.
I used to do a 'loose objects' check before flight, but such a check is never going to be conclusive since objects can become wedged in places out of sight and which do not reveal themselves in the customary 'slap the fueslage underside' check.