If by "form", you mean "school class" (used in BrE) then you could say "I'm in a good form" meaning that you like the class you're in, maybe the other children are well behaved or something.
If you mean that you are in a good mood or doing something well, then "in" is the wrong preposition. It's "I'm on good form".
Duly corrected. Sorry, for some reason that usage just sounded totally wrong initially when I read the post. However, when sportspeople talk about being in good condition for a forthcoming event (for example) they might say "I'm in really good form at the moment. I think I could win this tournament!"
"I'm on good form" has a different connotation, as in my previous post.
You probably haven't gained much. I've never heard it either (and never missed it). Though I would have assumed it was some American version (or from elsewhere) of "to be in form" if I did hear it.