Can I copyright materials I developed if it was asked to be written by the school I'm working at? I'm a teacher in a private school and was asked by the school to develop lesson plans for an existing text book.
Eventually that's going to be a manual for the teachers to use for that specific text book.

I am paid for my time doing this, however there are more things I was asked to help with during that the time frame I was given, mainly develop more materials for school.

The lessons plans can be used in any school that uses the same text book hence I would like to protect my rights and make sure it's not going to be used  in other different school without my permission.
Can I copyright my materials?
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.)