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:

The first edition suffers from being a victim of its own success. Much of it has become common wisdom and many of the refactoring techniques now have IDE support.

"Working Effectively with Legacy Code"^1 is usually what I recommend instead. It felt like a more up to date approach

More generally "The Pragmatic Programmer"^2 is a classic for a reason. But from you're comment you've probably already read it.