Glimpy Gang

Glimpy Gang is simply referring to a group that is smart when separated from each other but does stupid things when together.

(Not an actual gang, just a group of friends)

Person A: "That's the Glimpy gang"
Person B: "Don't they all individually have better grades than 90% of the school?"
Person A: "But when they're all together, they lose 90% of their brains"