The hypothesis that the skateboard mounted with the most expensive bearings travels the furthest is proven to be true.
The more expensive bearings are normally machined to higher precision and smoothness. However, t more precise bearings are also less durable. Therefore medium priced bearings provide a good balance between smoothness and durability as they don't break as easily. Also Consider:
This science fair project can be repeated using skateboards of different weights or designs.
Which ones travel the furthest? What about wheel size? How does changing the wheel size affect the distance/speed of a skateboard?
Build your own pro grade skateboard - http://skateboard.about.com/od/boardmaintenance/ss/ChoosingSkBoard_4.htm
Bearing (mechanical) - http://en.wikipedia.org/wiki/Bearing_(mechanical)
How bearings work? - http://science.howstuffworks.com/bearing1.htm
Basic safety requirements