Kid's book about smart kids

[rissabby]

A few years ago my grandson was reading a book about a very smart boy who was going to take some mysterious kind of test. When he's near the test center, he meets a girl who has lost her only pencil down a storm drain. I think getting it out is part of the test.

I didn't get any farther into the story, but I'd like to read the rest of it. I think it was closer to a kid's book than a young adult book.

Any ideas?