The situation you seem to have is that you have students (your volunteers) suddenly thrust into the role of teachers without any preparation other than what they can emulate from their own teachers. Teaching is a skill that can be learned but only through reflection and practice and that takes a lot of time. And you don't have that time now. I would guess that most of your volunteers are (a) used to be lectured to, and (b) have a learning style consistent with that.
However, as I've said in response to other questions here, the students aren't at all like their teachers. Every student is different and your learning style is not the same as that of your students. Good teachers learn this, mostly through experience, and learn to accommodate students with a variety of learning styles.
My advice, then, would be to try to have the volunteers give up a "teaching role" so that they don't get trapped in the emulation game. Instead, have them work with small groups of students in a responsive way, answering questions only rather than giving out information. Have the volunteers act more like mentors than teachers. In other words, divide the students into groups with one or two volunteers for each group. Each volunteer is just a resource for the group, not its leader.
One thing they can do, of course, is give something of their own backgrounds and what led them to tech. They can talk about what they find hard in learning and how they work to overcome that. They can talk honestly about the obstacles they faced (and that women in general face) and how those can be at least partially overcome. Counterbalancing the obstacles, the volunteers can also talk about what they love most about geekiness and what cool things they have built or helped build.
Mentoring relationships can be fostered and it would be good if you can find some way for those to continue after your volunteer gigs end.
But if they think of themselves as teachers they will just emulate the teaching they have been exposed to and that may not lead to the best outcomes.
If your volunteers know something about Agile Software Development process, especially Extreme Programming, you can do something like the following. Have each volunteer act as a combination Customer and Coach to a team of students. These are two quite different roles (two "hats"). As a customer, the volunteer breaks down a project into stories and feeds them to the team over a set of iterations. The Customer also handles acceptance of the work between iterations. As a Coach, the volunteer keeps the students on track, observes the periodic (daily) meetings and makes sure tests are written. If every team works on the same project, then you might be able to have one (or maybe two) volunteers act as Customer for all the teams, and the others serve just as coach (and mentor, of course), one for each team. The volunteers, by the way, will learn something if they also compare notes periodically and have their own debriefing sessions.