[EricH]

Could you get a district administrator or three to a meeting (or six) during build where these members are doing something (or supposed to be doing something)? This would kill multiple birds with one stone:
1) Continued/improved administration support, as they see what you're doing.
2) The administration sees what the problem is, and hopefully allows you to do something about it.
3) If the students in question see administration there, they just might work harder, even if it's just to look good.

The other thing is to make sure the students want to do whatever task they're supposed to be doing. If they don't, you can't really force them to do it. However, if they don't want to do any task, you could just suggest that since there isn't anything they want to do, they could go off in some corner and work on homework until there is something they want to do. A few rounds of that, and they might not be back.

Now, if they want to do a task and just get distracted, that's another issue entirely, and it's not one that is anywhere near easy to solve. It means the students have to learn how to focus. That's not an easy thing to learn.

The other thing to talk to the district about is, "Yes, I know we're supposed to be all-inclusive. However, these X students are using club meetings as a place to screw around in, instead of a place to build a robot in; that is, they aren't really being part of the club. This is hurting the entire club when we're trying to ready a robot for competition. Is there NOTHING we can do besides turn part of our workspace into a study hall for these students so they actually do something productive?"