As @David Heffernan said, don't worry about cross platform unless you must have it (premature optimization). There is a price to pay for that flexibility that most cross platform kits offer. Further, from my years of experience as both a developer and a user, native platform toolkits generally offer a better end user experience (often less surprises about what and how the user is allowed and not allowed to do with each control or surface).
In a learning environment, you want to consider what you hope to accomplish in a semester. A typical three month overall semester schedule with successive milestones suggests you'll want low friction. Learning a gui library can be an excellent achievement, but you'll probably want some basic familiarity with at least the environment or context before you get started in order to actually complete the intermediate tasks in a timely manner: presumably interactions and otherwise general user experience are at the core of the class' focus.
For these reasons, I would consider sticking with c++, as you have a sense for the paradigm and fundamentals of the language and tools. But, because it is a learning environment, I would strongly consider something less mainstream, but ripe with potential in the library. I might look at hardware accelerated windowing libraries, either OpenGl based xgl if targeting unix/linux, or Windows Presentation Foundation if targeting Windows.
