Events in my family's personal life mean that I will have to withdraw my child from your school. My work requires that we move away from the area and, as a result, it will no longer be appropriate or convenient for my child to study here. I am therefore writing to request a school leaving certificate for my child, so that they can continue their education in their new school through their home schooling system.
Obviously, this is just a sample letter. Feel free to use this as a template on which to base your own letter. You would need to include your own personal circumstances, of course, in order to make the letter legitimate and legally correct. The letter can also be written from your child's point of view, for example, "my parent's work requires that we move away from the area, so I am writing to request that you provide me with a copy of my school leaving certificate". In this case, the letter would be signed by your child, rather than by you as a parent.
School leaving certificates show that your child has passed the final examination of secondary school. They are necessary in order to pursue any higher levels of education your child wishes to embark upon.