Well, certainly a big o is when Mr. Keating forces Todd into creating a pneoem on the spot in front of the rest of the class. Although Todd is obviously very embarrassed, his eventual success, thanks to Mr. Keating's promptings, causes him to gain a new sense of confidence in his own abilities.
Secondly, I would suggest the scene where Neil gives Todd his present to throw over the side of the bridge where they are standing. This represents another stage in Todd's ability to resist and stand up to authority.
Lastly, I would say the final scene, when Todd is the first one to stand up on his desk and say "O Captain my Captain" to Mr. Keating as he leaves the classroom in shame and disgrace. This, if you like, represents the final development of Todd. He appears to have conquered his timidity and is willing to stand up (literally) for what he believes is right no matter what the cost.