[Chris is me]

The short version is you have to give them a reason to stay.

It's a really complex problem with no answer that's going to guarantee you 100% retention, but here are some places to start:

• Kids have to feel like they can do this. If there isn't enough help and they're expected to jump in on their own, they'll feel lost / incapable and move on. This is where quality mentorship really matters.
• Kids won't want to stay if they feel like they have to "wait their turn" to get involved. Telling new students they get to do the cool stuff only after a few years on the team is a very quick way to make your team very small. Maybe the freshmen won't have the same jobs or responsibilities as a senior, but they should be doing something of value.
• Whatever they are doing, they have to feel like what they did influenced the trajectory of the robot and really mattered in the season. Prototyping is a great way to give kids a sense of ownership of the robot ("my intake made it on the robot!!!") while retaining a strong sense of design direction from leadership.
• In general, kids need mentoring, either from older kids or from adults. Only the most driven and motivated kids will force themselves into their area of interest and demand to learn everything they need right away. Teach kids everything you can before the season so they can contribute during the regular season.