First follow this link, https://www.office.com/?auth=2&home=1
You will be asked to login, Use your full school email Account. e.g. test@chewvalleyschool.co.uk.
You should then be sent to your home page. Select the "Install Office" button in the top right corner, then select "Office 365 apps"
Run the file that should have downloaded,
Once it has finished installing it may ask you to login when you launch one of the Apps, if it does so, use your school account again.