Colo. Rev. Stat. § 1-12-105
A petition to recall a school district officer shall be signed by eligible electors of the school district equal in number to at least forty percent of the entire vote cast at the last preceding election for all candidates for the school district director position occupied by the incumbent sought to be recalled. If no such election was held, the petition shall be signed by eligible electors of the school district equal in number to at least ten percent of those electors residing within the school district on the date that the petition is approved as to form under section 1-12-108(4). In no case shall the number required for recall be less than ten percent of eligible electors qualified to vote in the most recent biennial school election; except that no more than fifteen thousand signatures is required.
C.R.S. § 1-12-105
For the determination of existence of vacancy in county offices, see § 30-10-105.