Lodged his balls in the robots' gears. I'm more curious how the invisible girl passed.

According to some Q&A with the creator, many of the robots have highly visible on/off switches on them, and rely on their cameras to find targets. All she had to do was climb up on them and hit the switches. Apparently it's an intentional design choice so that students who rely on stealth or precision have a shot.

Smells like a retcon to me, but I'll allow it.