[Bongle]

Remember that there are multiple demographics you can attract, and each of them care about something else. The college-bound want to pad their resumes and network, the social people want to meet more people and have fun, mechanically inclined people want a manufacturing challenge, business people want to interact with businesses, nerds want to design, strategize, program, and do math. If you emphasize the wrong thing to the wrong demographic, you'll fall flat. You could potentially prepare multiple posters, each targeted to a different subgroup, and post them in areas of the school most frequented by those targets.

Programming recruitment: by computer labs
Manufacturing: By manufacturing wing
Smart kids: Library? Guidance office?

For example, in high school my mechanically-inclined brother was the one who recruited me, and I thought the whole thing was a building competition, which was completely unattractive to me, because I was more interested in computers and whatnot. If it had been cast as a programming challenge that would help me build skills to get good summer jobs, I would have more readily joined.