Second life has been used to create a virtual classroom, but let's face it: Sitting in a (virtual or physical) classroom is not as much fun as playing Sims, Farmville, or even an old-style text-based advanture game.
I think the key to this is making the virtual learning environment (VLE) as interactive as possible. It is not possible to make it "a game" as it needs to provide an effective learning experience as well as being interactive. There are some Computer Aided Language Learning (CALL) programmes that use animation and videos to direct the students' learning but these require programming skills and time that I do not possess. Encouraging the students to interact in the VLE is vital to making the learning experience more immersive.
If Second Life were actually a planet we could visit, I'd like to think we wouldn't expend all that effort and technology only to replicate a classroom. Instead, we'd look at how the planet is different--different gravity and atmosphere (everyone in Second Life can fly, fall without injury and breathe under water), flora, fauna, cultures and subcultures, and consider what learning this new environment can support better than our planet.
We would also allow time for exploration by our colonist-students, and ask them to share their discoveries. We would, of course, have our own experiments and simulations which we've brought to our Second Life planet specifically to take advantage of its special attributes.
One way to increase engagement in a virtual world simulation is to assign students to avatars or avatars to students so that they are no longer representing themselves but another person or entity, a somewhat lower-tech version of the film Avatar. These could be used for role play, language practice, drama exercises, sociology experiments, any professional activity involving 'people skills'.