There are a lot of factors with skiing well at ski school that you can't duplicate at home.
First off you are getting expert coaching on each pass, so your bad little habits are not creeping back in. So by the time you ski your PB, you are really at your best.
Second - you are at ski school so all you are doing is skiing, getting coached, hearing others getting coached and talking about skiing - not driving, coaching others, rushing off to work, school, kids stuff etc.
Third - Conditions at ski school are usually uniformly good.
I would agree maybe they slow the boat down the first try or to get you over the hump on a particular pass or give you the ski school swerve, but you could argue that those are viable coaching techniques to build confidence. But you know what some/a lot of us are too stubborn to do that ourselves. I guess the part that you would like to know from the coach/driver is if they did slow the boat or help with boat path to get you your PB.
Then there is the whole discussion around whether you ran your "PB" once at ski school and now can't duplicate that at home, which I think falls into the "even a blind squirrel finds a nut discussion".