Instead of another vaccination, you can request a titer, where they will draw blood and test it for antibodies to any of the diseases for which your child received vaccines.
This is particularly important for the MMR, where they require 2 doses, but the booster dose is ONLY on the schedule because 5% of those who received the first dose did not develop antibodies. The other 95% are SUPPOSED to have lifetime immunity.
So if your child is one of the 95%, the booster dose is unnecessary. And the schools (and the doctors) are supposed to accept a positive titer result in lieu of vaccination.
You'll still have to sign an exemption for the other vaccines required for school, but the benefit of getting a MMR titer is that your children won't be excluded from school in the unlikely event of a measles or mumps outbreak.
I think all schools accept a MMR titer, and probably chickenpox as well. They don't accept titers for all the diseases. I know they don't for pertussis.
I think there's a good chunk of us on here who started off vaxxing on schedule or delayed/selectively and stopped for various reasons. You could look at it this way - if you had heart disease from a poor diet you could say "oh well damage is done" and continue on making the same choices, or, you could wake up one morning and decide to make a change for the better. Titering is a great starting point for change as you can physically see results and go from there. It's where I started with my dogs which then lead me to stopping vaxxes for my DD (that and some adverse reactions)!