Try Radio Shack. You'll need to schedule 'breaks' from the headphones to discuss where they are, what they're seeing, and ask questions. This isn't the best way to teach with computers, but it's better than not teaching with them at all. As a teacher who likes to 'co-explore' with my kids, however, I would feel frustrated in your lab. Parallel hard surfaces are the worst hard surfaces, -which is exactly what you find in most Sunday School rooms.
Volume is loudest when the sound wave has nothing to deflect it.
Aiming speakers across the room at each other creates half the problem, try angling them away from others. Distance helps, so does placing a discussion area in the middle. Not all soft materials are acoustically soft. Certain types of foam won't reduce much noise. Think of the eggcrate walls they used to have in old sound studios.