Cobol by itself is quite simple, it’s the whole code and architecture that is convoluted.
I agree with the others - do whatever you think is fun and inferesting at this age. Too many things will change before you get to the job market.
Having said that, reading up about Cobol can be fun and interesting, and unique.
I can also recommend this book about working with legacy systems. It makes for an interesting read:
(And also other classical books like Pragmatic Programmer etc)