Reposted (I have no idea why my original was posted as AC while I was logged in?!)
I've seen enough "if they're smart, off to college!" type responses here and elsewhere to know that it's a fairly popular proposition. However, as someone who did just that - I left high school as an under-18 kid to enroll in a very demanding engineering faculty in a prestigious university - I can attest that the transition can be extremely difficult. I simply didn't have as mature a mindset I needed as well as the social skills to easily succeed that early. Eventually, I did graduate, but for the first couple of years, I was very close to simply dropping out (and I knew enough colleagues who did - which was a complete waste of their talent and knowledge). I know anecdote =! data, but high school allows a child to struggle and fail without some very real consequences (mostly having to do with the already high and growing cost of retaking courses in university and/or continued residency, or not being allowed to proceed in a field due to low marks, etc).
The Ontario (Canada) HS system used to have an "extra" year (Gr13 or OAC) that was abolished some years back. As someone training to become a teacher (thus, my nick), I've already observed some very obvious negative trends (from talking to and working besides teachers who've been on the front lines for the past 10-20 years) due to the loss of a school year. Without the extra year of prep, students interested in university are discouraged from taking courses outside of the core curricula necessary for entrance. Sadly, this means stuff like comp sci courses, which used to have packed classes, are now sparsely attended and are close to being removed (if not already gone) at many high schools. Other things like integration in calculus (something OAC math used to have) have been dropped for parity with students coming from HS boards that only go up to 12 (who don't teach it) - leading to students being behind the eight ball almost immediately upon walking into any high science or engineering math course.
These two factors (amongst others) can lead to a situation where your high achievers, the ones who are so glibly asked to "go to college!", are negatively impacted by timing pressures or the attitude that they can succeed purely on academic terms.
I don't know anything about this mayor, so I don't know his politics or whether or not this is just a thinly disguised cash grab (as some have implied), but extending HS is not such an evil thing in and of itself.