Reading that, I don't think you were being rude at all, it was polite and informative, I would assume since it's a class thing that your teacher would want you to make a comment and use your brain and knowledge while doing so, which it seems you did.
hurtloam wrote:
Your Mum maybe thinks you were being too patronising with the "I don't know if you knew" phrasing.
That can be read in two totally different ways, until you mentioned it I read it the other way and would not have seen the second way.
Like "I don't know if you knew" ("let me correct you")
OR how I first read it; "I don't know if you knew" ("forgive me if I'm repeating something you are already aware of here")
Maybe you are right and it got read the other way when the intent wasn't that way.