Senior portraits do not have to be ordinary. They should be whatever you want them to be. Shop around. If your school's official photographer is the best, then go for it! But do your homework. Whether you are a student or a parent, you know that senior pictures are a once in a lifetime event, so why let that event be ordinary?
Use the internet and look at the web galleries of photographers near you. Call them or email them and find out their rates. But know what you want and go get it!
Ask them about their packages. How long will the shoot be? How many shots will you have to choose from? Can you have multiple outfit changes? How many finished images come with their packages, and in what form? What about Facebook/MySpace-ready jpeg files? Can you get more prints later?
Be a good consumer, do your homework, and you'll be happier in the end.