"Did you really blow up your Aunt, Harry?" said Hermione in a very serious voice.
" I didn't meant to," said Harry, while Ron roared with laughter. "I just-lost control."
"It's not funny, Ron," said Hermione, sharply. "Honestly, I'm amazed Harry wasn't expelled."
"So am I," admitted Harry. "Forget expelled, I thought I was going to be arrested." He looked at Ron. "Your dad doesn't know why Fudge let me off, does he?"
Harry Potter and the Prisoner of Azkaban
I don't understand how "forget expelled" is used here.
If the writer dropped the subject "I"(Harry), then the verb has to be "(I) Forgot", not "forget". But it still doesn't seem right. Doesn't it have to be "Forgot (about) expulsion" or "Forgot about being expelled" or something like that..
Or is it an imperative? Then how come "forget" is followed by "expelled"? I don't know what it means here.