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Dave uses the idiom "I was shaking like a leaf in the wind" to describe how he felt when his Pa confronted Professor Herbert.
Pa didn't understand the teaching methods and activities that Professor Herbert was using at the high school and didn't feel that field trips and collecting live animals were appropriate types of studies. Pa was also angry that Dave had been kept after school, thinking that the punishment was unreasonably singling Dave out because his family was poor.
"It jist don't look good to me," says Pa, "a-takin' all this swarm of youngins out to pillage th' whole deestrict. Breakin' down cherry trees. Keepin' boys in atter school."
And, Pa was physically much larger, stronger, and inclined to act out his emotions than was Professor Herbert. Dave was very afraid of what Pa might do to give vent to his anger and feelings about the school and Professor Herbert's teaching methods.
I thought Pa was going to hit Professor Herbert every minute. He was doing all the talking. His face was getting red. The red color was coming through the brown, weather-beaten skin on Pa's face.
Dave's body felt like it was shaking due to the fear and nervousness Dave was experiencing as he watched and listened while Pa and Professor Herbert were talking.