How often can you fly as a senior in high school? I know my senior year I was out by noon most days so if you can afford to do it I bet you could do most of your instrument/commercial training your senior year. I am not sure if the FAA has rules regarding being 18 to take the instrument and commercial checkrides so that may slow you down a little- but you can probably find the info about that on their website.
Next some of the regionals have an age requirement of 21 or 23 so look those up too- otherwise you might be working as CFI longer than you anticipate.