I did the same thing for a group of middle school students back in 2005 and after evaluating a bunch of graphics and sound libraries, we settled on Basic4GL.
Basic4GL is everything BASIC was, except without line numbers and with all the GLUT functionality built in (minus the initialization cruft). It also supports sound, loading a bunch of texture formats, and has the NEHE tutorials ported to it, and runs on VERY low end hardware. Download and run the demos -- you'll be impressed.
The kids did exceptionally well. We got a classroom full of (failing) middle school students to understand the idea of a coordinate system, and use this to design their own spaceship (using only a piece of graph paper and their own derived x,y coordinate pairs). We then guided them through animating this spaceship with key press events (and in the process they learned about coordinate transformations).
Our goal of having them design their own textures and sound effects never quite panned out, since we ran out of time -- but our ultimate goal was a classroom produced game where every student had a piece of the production workflow.
Afterwards, I found myself using Basic4GL for OpenGL prototyping since it does away with so much of the initialization, etc.
For example, the following is a whole Basic4GL program to draw a triangle
glVertex3f(0, 10, -30)
glVertex3f(8, -4, -30)
glVertex3f(-8, -4, -30)
This was, of course, several years ago. You may find something better now (I'd recommend looking into Processing. I'd stay away from anything that a kid can't set up on his own (i.e., combination of multiple libraries)).
For the classes, you want to emphasize the basics while at the same time giving them something they can sink their teeth into from Day 1. I started with having them type in a very simple program in the first class and then run it themselves. I went from there to what the coordinates mean, etc. You will find that some kids are faster than others, and some of them might surprise you. You will also find that they'll do really well teaching each other.