Just kidding -- go for it and lead the group. As long as good people and processes are in place, I don't see why a teacher at the same school could not be president, either.
If you don't expect too much from me, you might not be let down. <img src=images/smilies/smile.gif>
I would think if you are in a separate school it would be no big deal as your role within your school is "parent". It may cause you to be privy to some things however, that would be inappropriate to share so that would have to be a ginger line you walk.
If a teacher at the same school I would question why no parent stepped up but seeing as you are one, being a teacher might lend you a lot of insight into what is needed, when and where. Exploit the opportunity and your inside knowledge to make your PTO tremendous.
I am starting up the PTO at a brand new elementary school. I am a teacher within the same district. Is there a conflict of interest with my being a teacher and being the PTO president?