The schools, which include at least one feeder primary, were supposed to have supplied the Electoral Reform Ballot Services with the names of parents
eligible to vote more than two months ago.
They have now been sent further letters requesting the information needed to set the number of signature required on petitions to trigger ballots.
Nick Goodman, ERBS senior ballot adviser, expects the information "in the next few weeks".
And a spokesman for the Department for Education and Employment said: "If these schools don't respond we will be sending them a warning letter about their legal obligations."
Anti-selection campaigners seeking ballots in Kent and Birmingham do not know the size of the petition this requires.