We had this problem last school year in our elementary school. All the board members were 5th grade parents and wouldn't be around any longer. After begging, pleading (you get the idea) and sending notes home to parents with no success, we finally drafted a letter saying that the PTO would cease to exist unless parents stepped forward. Much to our surprise, we had enough volunteers for a full board and they are doing a fabulous job this year. There has been a lot of hand-holding as they work through their first year butnothing more than that.
You may find that unless you find a way to stay on next year (if you even want to), you may have to send home a similar letter pointing out all that your parent group does for the kids. Good luck!
I know in ou PTO we had that problem several times including this year. Our bylaws say the same thing. What we did was just switched seats. the president became vice, the vice became president, etc. just be careful though, sometimes if you get a "bully" parent who wants to start trouble, they can behind you back.
When we wrote our bylaws, we referred to the California Corporations Code. Regarding term limits, the CA code says "no more than three years, or until replaced by a qualified person." (I'm just paraprhasing!) Anyway, you'll probably want to check your state's corporations codes to see if there is a similar provision. Maybe you all can remain in your positions if you want to.
We're also facing problems filling board positions for next year. If no one else steps up, I say stay where you are or do some switching within your board. If no one else is willing to get involved, then there should be no complaints if the current board continues to run the group.
We have been unable to find any parents who want to run for our open board positions. Our by-laws state that the maximum term for each board member is 2 years. Our President (me), Vice President and Secretaries are all termed out. I've talked with parents after school, before school, at our family events...no one is interested in running for a board position. We don't have problems getting parents to volunteer for events...attendance at our meetings is really good (10-15 parents)- but, I stopped sweating attendance years ago!