Ours is almost entirely student built. Our class broke up into subdivisions (drive system, programming, special teams [which built the extendor], pneumatics, etc). The students designed the whole robot and formed strategy. As the class manager, I must say that this process was costly since some things didn't work and had to be rebuilt several times (like our $800 extendor), everyone learned a great deal from trial and error and how to fundraise
Our team captain even missed 3 weeks of school to build all day, something I don't reccomened personally but he had lots of dedication.
Of course, like someone mentioned before, a company did our welding because of liability. We also had one mentor from JPL but his main job was to nod if we were doing something right or shake his head if we were way off or doing something dangerous.
But the rest was student built and it was worth it even if we had to work from 9am-midnight on weekends and after school on weekdays. Our teacher who basically delegates power on our team didn't even know what our robot looked like until our first scrimmage, which was a day before we had to ship it off. He says its the best class he never taught.
I'll definately miss this next year.