Besides practicing in front of your team mentors, try and find some adults you DON'T KNOW - you may be comfortable in front of your mentors, but uncomfortable in front of adult strangers. Besides, what might make sense to your mentors, because they know all about FIRST and your team, might not make sense to strangers. Be prepared to answer any questions they might have.
Also, consider contacting your local
Toastmasters club for assistance. Be sure to read their
Ten Tips for Successful Public Speaking.