First off-- transformed battle Junketsu, or in the standard uniform form?
If you're going with the standard uniform
, my recommendation would be to make it as a separate piece entirely, and then just attach it via the epaulets. It's sort of similar in shape to what I did here
, and you could do something similar by adding a higher collar piece (using interfacing to make it stiff/stay upright around your neck) to a standard sailor fuku top pattern.
(For future reference, please post a photo of what you're trying to make so we can better help you! That way people who haven't seen the show or don't know which particular variant you're doing have a clear picture of what you have in mind).