[Stephen P]

That is a predicament, but not an impossible one. Try explaining to the teacher that the seniors' roles are to be team leaders. They are members of the team in all respects, and for the reason that they understand the build proccess and are veterans of FIRST. A senior's role is to lead BY partipating so that they can pass down the knowledge to younger members. My team is going through a similiar proccess right now, in that our senior members (myself included) will be departing with much of their knowledge next year. We are trying our best to incorporate the younger members into the design process, but we're definitely not sitting aside and letting them take over. Heck, we seniors have FIRST dibs (play on words there).
It seems that this teacher is very unwilling, but if you and the other seniors sit down with her and explain the circumstances, you might come across some common ground. If all else fails, look into getting a different faculty sponsor if possible. Have a meeting with the sophomores as well. It would be a shame to have a predicament like that ruin your last highschool year of FIRST.