Most of the teams I work use a press fit Aluminium tube system like this one
http://www.mcmaster.com/#press-fit-f...ystems/=5bymjz
Systems like this are light-weight, fast to assemble, easy to modify, strong, and relatively cheap. I wish all my teams would use something like this.
I think welding is a great and fun skill for kids to use but it can be prohibitive to use during the build season if the welders don't have much experience.