I saw this idea while doing something completely non-teacher related online...but I immediately thought of you all. I mean, what a GREAT way to hang a curtain in your room when you are dealing with concrete block walls or a 'no holes in the wall' policy.
There wasn't a great explanation on the site where I found this, but I'm pretty sure it's a brushed nickel Command hook with a tension rod (maybe like this one).
You could do a cute little valance over the window on your door (or the hall window next to your door) or even take this simple idea home and do some low-maintenance decorating.
Oh! And since it would be so easy to change the curtain out, you could change them out for each season or holiday. Cute!
Do you have a simple decorating solution for your classroom?