Only the Tech Centre Staff can Upgrade your Computer to El Capitan. You must make a booking with the Tech Centre for them to upgrade your computer.

1. Ensure you have at least 15 GB of Free Space on your hard drive. There are instructions on how to check this in this FAQ, How do I clear space on my Hard Drive? 
    
2. Email service@scotch.wa.edu.au to arrange a time to upgrade your computer. In the email Say That you Wish to Upgrade your Computer and include relevant details such as times that you are free (often lunchtime or before school may work well), be aware that this process may take up to an hour. The Tech Centre will respond to you by email to confirm a time.
    
3. Come to the Tech Centre at the arranged time and they will upgrade your system for you.