Charlie says in the episode he pretends to be sick to become a Cackle's student he likes HB so that part isn't random. Basically just the missing conversation between them on way back to the castle.
HB Is Funny!
Mildred concentrated on flying straight and tried to ignore Ethel's squealing as Charlie held onto her in duck form. Suddenly though Mildred heard Charlie laughing.
"What's funny?" Mildred asked him confused.
"This is the best day ever, the most interesting thing that happens in my school is an argument in the canteen" Charlie commented as he laughed.
"Yes well, you won't be saying that if HB catches us" Mildred replied, more worried than amused.
"And then there are the subjects, Maths, Science, English, none are interesting like they are with magic" Charlie went on holding duck Ethel tighter as the broomstick wobbled.
"I know, I went to normal primary school, I was never good at them either" Mildred admitted.
"Really? That's awesome, I thought all the students came from witch families, maybe I still have the chance to be a witch too" Charlie replied amused.
"Just be grateful that your school doesn't have HB" Mildred told him the bad part of the school.
"It has teachers like her though, plus I like HB, she's funny" Charlie gave his view.
"I think we have different ideas as to what is funny" Mildred replied as she saw the castle come into view.
"Try not to land in a haystack this time" Charlie commented.
"Oh yes I am sure you could do better" Mildred responded as she went in to land.
Mildred landed just missing hitting the broom shed. The door opened and Mildred and Charlie froze to the spot, expecting to see Miss Hardbroom, but they relaxed as they saw it was only Maud, Ruby and Jadu.
"A duck? What have you done?" Maud commented.
"I cast the wrong spell, I knew that reversal spell from by heart I don't understand" Charlie replied.
"I will go look it up in the library, you best keep Ethel out of sight" Jadu told them as the friends all rushed in opposite directions, avoiding the teachers.