The correct grammar would be "is".
You will hear people say "are", but it's because it's awkward.
Think of it this way:
"One boy is a good student"
"One boy in three is a good student"
"One in three is a good student"
You'll run into this with "each" or "every" as well, since they often have plurals between them and the verb:
"Each of the sailors has a white hat"
"Every one of the girls is clever"