I’m not sure that the clean sheet issue is particular relevant in all honesty.
Firstly the trainee is the one who did all the work to get the clean sheet. Some will take to it better than others and pass easily, others won’t be so fortunate. Although the quality of the training is important there is more to it than that. Some will get a clean sheet/good pass even with a poor trainer. Some will fail or get many minors even with a great instructor.
Secondly if a trainer wants to take credit for a clean sheet then by definition they would have to accept the blame for a fail or a poor pass.