That's excellent advise -- as much oral and written history (the good, the bad, and the ugly) that each of the outgoing board members can pass along to the newcomers, the better equipped they will be to continue the board's momentum.
Despite best efforts to follow by-laws, there will always be glitches that eventually compromise the intended staggering of experienced vs. novice board members. Sometimes it happens with mass resignations, other times it's coincidental when one board member then another over the years has to resign mid-way through his/her term and it eventually catches up with the board with all vacant positions at the same time.
However, you may want to consider adding a couple more board positions with one year terms just to try to provide some balance so that the situation is less likely to recur in 2 more years. Also, while your outgoing board members aren't interested in renewing their terms and all the responsibilities that go along with those positions, they may be willing to serve on committees for one or two of the activities that were under their umbrella of responsibility during their terms. That way, the new board member has experienced help to draw upon during their first year while learning the job.
I'm not sure I have much in the way of help, but I stepped into an entirely brand-new board this year so I'll offer you a little perspective from this angle. Make sure you leave the new board any and all information that you have. We weren't given anything and we are flying blind. Start a notebook for each position and list things you've done, what has and hasn't worked, pointers, even your contact info in case the new chair has questions. I'm fundraising co-chair and I can't even get anyone to tell me what they did last year (or previous years) much less whether it worked or not - so frustrating and it is the kids who will suffer for it in the long run!!
Also, if I were in your position I think I would figure out a way to change the bi-laws because otherwise it sounds like you're going to continue to have this same problem. Whatever happens, good luck!
All four of our PTO Board positions will be open this Spring. Our bi-laws state a 2 year commitment (has been met) with a staggering format (not met). When all four of us started the previous board all left at once leaving 4 positions open. Now, we have the problem of not being staggered. I am the President and cannot fulfill another year. I am certain the other 3 are not looking to serve on the board again in the spring. I really am feeling bad about trying to fill these positions when all 4 are open with nobody left to help balance out the experience. My biggest fear is that nobody will volunteer to help serve on the board. We have had a very successful PTO and would hate to see the momentum end now. Any suggestions on how to be proactive on this challenge? Would love the feedback.