Copago, that's fab. I think your pupils are very lucky.
The only thing I could think of was an art activity for them -- you could make Narnia scenes in snowglobes. If you haven't tried before, google "how to make a snow globe" and read several of the results because there are different tips to be found, including using baby oil instead of glycerin, which I wish I'd thought of when I once made one with my daughter. Maybe it's a bit too last minute a suggestion, though, as it requires figurines etc. Paper snowflakes, where you fold the paper and cut intricate patterns, are remarkably satisfying to do too, I find; and it would open up the discussion into a bit of science (states of water, ice molecule structure something or other -- if you don't mind I might ask for your spark notes if you teach that lesson, lol.)
I don't know if you have younger children to teach too but I've made great use of this site when trying to keep nephews/nieces occupied: http://www.activityvillage.co.uk/narnia.htm
- (they might be the same word searches as Ren's).
You probably already know the site, but if not it is worth exploring as it has a wide selection of activities/inspiriations, ooh I just found these outline maps, for instance. http://www.activityvillage.co.uk/outline_maps.htm
And look, printable frames - picture frame & stamp - http://www.activityvillage.co.uk/royal_frame_printables.htm
- they could design a stamp ... think I'll print that out for my niece. It's the site that keeps on giving.