We have this same situation. I published a set of speed tips that the scout leaders distributed. Some very helpful tips about orientating the block correctly. I mentioned that extending the wheel base can help as well as canting the wheels and mentioned that colored wheels were available at the scout shop. To use graphite and not white teflon. Where to balance the car. That was about it. Sanding axles as well.
It acomplishes two things. Kids will all finish the race and there won't be as much controversy when you show up with blue wheels and an extended wheel base with canted wheels, etc...