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:
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.
- 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.