Is the article necessary in this collocation?
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".
There are over thirty examples of "be + in good form" in the BNC.
"I'm on good form" has a different connotation, as in my previous post.
Thank you, it was the "sport" context which I meant. It was also good to learn the collocation "to be on form" which I never heard of. ))