So what is the most important question to ask your senior photographer prior to your session?

At this point, you should know your photographers style and their work quality - which is very important when selecting your senior photographer.  Now, you want to ask for their guidance with wardrobe selection.  Photographers know what colors and styles that photograph best.  They will also know the best way to maximize your session time and also get as many looks in your scheduled time as well.

My best advice for all high school seniors?  

Don't just trust anyone to photograph you for your senior pics.  Make sure they are the photographer that can bring your visions to life.