Well, I would highly advice you to have a look at play framework
Have a look at the video on the homepage
It has a gae module
With siena as a orm to handle gae datastore
From all the java frameworks I've seen so far, I think it has the easier learning curve,
great documentation,
a tutorial to develop a complete application
very active and helpfull community,
free as in beer and also as in speech
highly scalable due to it's stateless out of the box design
rest friendly
highly customizable via it's modules and plugin approach
more over, taking into account it's for a college course, it's really easy to hack into the code, I could handle a couple of tickets and new features without any experience at all with java, just a couple of year developing web applications with php
other benefits
it's really easy to start, no complicated setup involved, just download and unzip the file and your are ready
great development experience, just fix the code and hit refresh, play autocompiles changes on the fly when running on development mode
fast and lightweight
fullstack, you don't have to integrate cumbersome frameworks to make it work
great errors report, no more endless error stack trace, just shows you the line with error
no xml configuration anywhere around
great support for tdd with selenium integrated test
I think the main advantage as a learning tool, is that it's possible to peek into the source code without feeling lost and the community support... The documentation is very clear, and you can learn a lot by just having a look at the source code...