A person holding a valid credential authorizing substitute teaching may serve as a substitute for the appropriately credentialed special education teacher as follows:
(a)
Except as provided in subdivisions (b) and (c), the employer shall not employ an inappropriately credentialed substitute teacher for a period of more than 20 cumulative school days for each special education teacher absent during each school year.
(b)
Upon application by the district or county office, the superintendent may approve an extension of 20 school days in addition to those authorized by subdivision (a).
(c)
Only in extraordinary circumstances may an additional number of days be granted beyond the 40 school days provided for by subdivisions (a) and (b). Such additional days shall be granted in writing by the superintendent. The superintendent shall report to the board on all requests granted for an additional number of days pursuant to this subdivision.