You can, but she might take her case to an employment tribunal.
The Green Book, which deals with local government employment, was revised in December 2000 to cover the issue.
The relevant circular stated: "Authorities should consider the full range of flexible working arangements and support facilities for employees returning to work."
There is a strong disposition in favour of meeting the needs of mothers returning to work, without it being an absolute legal requirement. If you want to gauge an employer's chances of success, consider the job-share headship judgment.
Every case is judged on its merits, but you need a good case. If you are an LEA school, you will also need the authority's backing and that may not be easy to get.