The book never says that Darry is "super smart." However, there are two places in the book when Ponyboy comments on the fact that Darry is smart. Page numbers will be different from edition to edition, so I will give you the page numbers that I have. I will provide the full quote too, so that should get you really close to the exact location and page number.
The first time that Ponyboy comments on Darry's intelligence is on page 7.
He looks older than twenty -- tough, cool, and smart. He would be real handsome if his eyes weren't so cold. He doesn't understand anything that is not plain hard fact. But he uses his head.
I do not really think that is a glowing endorsement of Darry's potential intelligence. Ponyboy says that Darry looks smart. That doesn't mean he is smart. Ponyboy does clarify himself a bit and says that Darry "uses his head" This tells me that Darry thinks things through, which I believe is a smart personality trait. Unfortunately, Darry doesn't seem to have any creative thoughts. He only deals in facts.
The next smartness quote about Darry is on page 108.
"I know" I said. I had known it for a long time. In spite of not having much money, the only reason Darry couldn't be a Soc was us. The gang. Me and Soda. Darry was too smart to be a Greaser. I don't know how I knew, I just did.
This time Ponyboy's thoughts do convey the sentiment that Darry is most definitely smart. Ponyboy admits that he doesn't know why he knows that Darry is smart, but Ponyboy definitely knows that the Greaser gang is holding back Darry's true potential.