Juan21 wrote:(Is this a good idea?)
Probably not. They'll learn more if you focus on just one platform.
There seem to be thousands of options: PICs, AVR32, 68HC11, 8052, etc. I've tried to search for microcontroller comparisons, but they seem to be either inconclusive or just commercial hype.
Assuming you're actually going to have them use hardware, rather than just read about it and try to memorize facts for a quiz, they're not going to spend their time with the actual microcontroller details. That stuff really don't matter much.
What does matter is the software and programming language/environment. The method of physically connecting the parts (eg, a solderless breadboard) also matters greatly. Those are the places where they will actually be working. Even if you're going to have them use special on-chip feature or peripherals, ultimately they're going to do so by physically connecting stuff together and actually using software on computers.
You may feel like I haven't answered your question? But consider if you were going to teach PC computer skills? Would you focus on CPU speeds, hard drive sizes, types of graphics cards and other specs of the innards inside the PC? When you go buy a computer, that's the stuff they tell you about. Likewise when you shop for microcontrollers, comparisons give you lots of specs on the internal capabilities. But just like with ordinary computers, when you actually go to use the machine for something useful, while those many specs do have some impact on how effectively it can perform more demanding tasks, you generally don't focus on that stuff while you're actually using a PC to do anything useful. Especially if you're doing simpler stuff that doesn't push the hardware to its limits, which seems likely at the high school level, all those tech specs and internal details of the machine really don't matter greatly. What does matter is choosing easy-to-learn software and focusing on achievable tasks or goals, so the net learning result is some useful experience actually using the whole system to accomplish something.