agree with lcb123
Look for the person specification in the job pack. See if you can more or less say something about each part of the person specification. Group things if there are overall themes.
So the letter can go:
Dear xyz,
Application for School Administrator
Enthusiastic sentence about why you want the job (eg, I would love to use my skills to contribute to the efficient running of the school and help all the staff, children and parents to feel supported)
Sentence/paragraph about first aspect of person spec (eg, I am experienced in word, excel and databases from my time at abc job)
Sentence/paragraph about next aspect (eg, I thrive in a fast-moving and multi-tasking situation, and I am good at reacting quickly. Not only did I respond quickly to unexpected situations in Xxx job but looking after my own small children has honed my skills in multi-tasking.)
continue until name checked all/most of person spec- though keep the whole letter to 1 page ideally
Finish with similar enthusiastic sentence to the first one, something genuine and positive, or just say I hope you will consider me for this role,
Yours sincerely,
BlastedPimples