An opinion from a girl just out of high school (Score 1)

What you do depends on how much IT knowledge you're expecting the kids to have coming in. Not many schools teach robotics, but do teach programming or web design. So a robotics competition might not be the way to go, but a programming one might. Some places to check out:

• Nebraska Educational Technology Association Student Contests - I'm sure other similar association exist, but this is the one I'm familiar with
• Northwest Missouri State and University of South Dakota both held programming contests for high school students with a good turnout.
• Science Olympiad has some technology-themed events that high school students can enter, including two robotics competition. But from my experience, not many schools participate in those competitions.

And these are just some things that I alone have had experience with. I agree with those who say that you should contact local schools and see what they teach and how many students are intersted in them and base decisions off of that.