Why would Whitey get into the band or orchestra try out by only playing scale admittedly smoothly worn 6 months of lessons, whereas Beaver who had supposedly not had any lessons was made to try out with playing part of America?
I was of course part for plot.

Yeah, for plot. They got some good comedy out of Beaver playing only part of it for Ward and June.

Kind of like when Larry learned only part of the tree poem. (Miss Landers: Larry, you were supposed to learn the whole poem. Larry: We had company and I had to keep coming down and meet them.)