When I photograph High School Seniors, as with all my subjects, I want to capture their personality. It isn't easy for me to evoke emotion in a portrait sitting, so I try to make their experience fun and relaxed. Maybe ask them to twirl, jump, lay on the ground, hang from a tree.
I want to see attitude from the senior, if that is something I can get them to demonstrate. As long as it's a natural look, it works. This is something that cannot be faked. I don't want them to look angry, just confident.
I don't purchase props to use in my photography. If I were to use props, then my portraits would all start to look the same. Instead, I ask the student to bring their own, if they desire. I encourage them to bring an item that represents what they are involved in for school such as sports, music and art, or something more personal like a hobby or other interests. This is for them, not the parents, at least in my way of thinking during the session.
For me, it's about capturing the senior as they are in this time of their life. This could be the last time they are working with a professional photographer before they enter adulthood, possibly until they are engaged to be married. I want it to be a great experience. Every person is different, and it's up to the photographer to capture each subject's unique personality and look. Don't try to force everyone into the same box. What works for one, may not work for another.