i dont think this is true.
there is an art to educating (especially the ~10-15 year old range) that does not just manifest itself because you are smart: how to engage students, how to keep them engaged, how to adjust the message to the audience's level and communicate it effectively (which changes kid to kid), how to earn a kids respect without becoming over-bearing (or too friendly), and dozens of other things that your PhD in compsci or whatever does not teach you.
some of the smartest PhD holders i know would be very shitty elementary/high school teachers.
(context: i teach at the college level. its a lot easier than teaching at the high school level.)