The training session takes three days. During that time the tributes are to meet experts in certain skills and hone their own skills. Katniss and Peeta start with knot tying, learning to make traps, and then move on to camouflage. Katniss says,
"We do pick up some valuable skills, from starting fires, to knife throwing, to making shelter." (pg 96)
The second day they notice that Rue is shadowing them. She starts to join their training sessions. Katniss notices that
"....she's clever with plants, climbs swiftly, and has good aim. She can hit the target every time with a slingshot. But what is a slingshot against a 220-pound male with a sword?" (pg 99)
The third day each tribute must demonstrate a skill individually to the Gamemakers. Rue is called into the arena, but we never learn what talent she demonstrated.
Although the Gamemakers visited the training sessions, and some took notes, it is never explained how the scoring was done. Since all the tributes were first learning some of these skills, it is doubtful that the scoring was based on these sessions. It is more likely that the scoring was based on what they demonstrated during their individual performance (i.e. Katniss's demonstration of her bow and arrow skills).
We know that Rue received a seven,
".....an excellent score for one so small" (pg 126).
However, the only hint we get as to what skill she demonstrated is when she said,
"I'm very hard to catch.... And if they can't catch me, they can't kill me. So don't count me out." (pg 126)
Katniss does not know why Rue received her score.
Despite the fact that Ru is the youngest thus targetted as the youngest and weakest tributeby the others, her score given by the game masters is relatively high. How tributes get their score is a highly guarded secret: Haymitch and Peeta both advise Katniss to keep her archery skills to herself in the practice rounds. This is supposed to give the tribute a tactical advantage in the arena, also making enjoyable viewing for the citizens of the Capitol. Up in her tree, Katniss is able to witness Ru's survival skills at "tree flying" and evading the other more brutal tributes, making the connection that these are the skills that must have surprised and delighted Seneca Crane and the others watching.