Can I copyright materials I developed if it was asked to be written by the school I'm working at?
As I understand it, in the UK the answer is that your employer owns the copyright, unless they have told you otherwise:


  Where a literary, dramatic, musical or artistic work, or a film, is made by an employee in the course of his employment, his employer is the first owner of any copyright in the work (subject to any agreement to the contrary).


https://www.gov.uk/guidance/ownership-of-copyright-works#works-created-for-an-employer

(Note: the question 'can I copyright the materials?' doesn't actually make sense. Copyright automatically exists on the creation of a work. Your question is really about who owns the copyright in this case.)