Many high school seniors (and parents) choose to capture the momentous time in the adolescents life with a professionally done senior portrait. Think of this as yet another "rite of passage" so to speak, that almost every 17-19 year old takes part in. These days, many high school seniors are looking for a more creative way to represent this time in their lives, so they opt for choosing a private photographer to do the shoot.
If you're looking for a photographer to do your senior photograph, remember, establishing a relationship and a certain level of rapport is the most important thing you can look for. If the trust and openness doesn't exist, the pictures may suffer. Also, it may be helpful for the senior or the parent to bring a list of ideas they can discuss with the photographer regarding location, clothing choice, style, overall look, etc.
It is the senior photographers job to make sure he is helping the student to achieve the look and feel they want to convey while still pleasing the parents, who are probably paying for the picture. Finding this balance can be difficult sometimes, but the seasoned senior photographer will have traveled down this road many times before, and know how to handle most situations.
If you're not looking for your own senior photographer, most high schools now hire theyr own photographers who probably have extensive experience in taking the most flattering and expressive shots for each student.