I would strongly recommend something my team did a number of years ago - the "No Mentor" line. Let me explain...
As a team, we had come a long way, and it was the first time we had a significant number of students on the team who had been there for 2-3 years. The captains approached us and told us how much they appreciated everything we had taught them over the past few years. They went on to tell us that they felt it was important they take the next step at competition this year - they knew everything there was to know about the robot, and were confident they could keep it running without us. So, they were implementing a new rule called the "No Mentor" line, with the line being the tape outline of our pit. They didn't want mentors in the pit during the competition so they could prove to themselves and everyone else that they had what it took to do this themselves. They still wanted us at the competition, and they were clear that the line could be broken IF they specifically invited us across to help with a problem. We were to be their safety net - something there if they needed, but hopefully something never actually used.
Due to that decision they made, myself and another mentor started volunteering at the regionals as inspectors, but I digress.
Try a similar approach to show everyone that you guys have what it takes. The key really is all in how you present it - you aren't implementing the "no mentor" line because the mentors have gotten overbearing. You're implementing it because they've done such a good job teaching you everything you need to know that you want to prove your capabilities to yourselves, to them, and to the rest of FIRST.
Hopefully that gets you through the rest of this season... looking forward though, you need to come up with a document to govern your team. That doc should clearly define the roles for mentors and students. Make it clear in the document that the team captain (or captains) is in charge, and pick a strong captain for next year that will work with the mentors to better define the roles each should have in the design and build process.
I know with my team we (mentors) try to provide as many options as we can so the students can make an informed decision. It's difficult not to attempt to steer them towards my preferred option, but I mostly get around that by playing devil's advocate - Basically, I try to take the opposite position for any design decision up until that decision has been made. This year, that was mostly revolving around our method for pulling down the catapult - I think we had 8 options listed on the board, and it was a very difficult choice for the team to make. but given the process we went through, I'm confident that the students involved know exactly how each method would work, they understand the pro's and con's of each, and can clearly articulate why they made the decision they did. And really, that's the whole point of the exercise
