If a certain person was in the photographed and showed you it, that would be considered a primary source because the person is showing you are photograph of him and his classmates. If the photograph is in a textbook or was shown by someone else talking about those classmates, then that would be considered a secondary source because they were passed down that image. I'm assuming you were shown this photograph in a textbook or by a teacher, so I'm willing to assume that the source would be secondary.