Google Vault stores your schools data even if a user deletes and empties their trash.
Sign into your google admin account at http://admin.google.com
Click Apps
Now click G Suite
Select and click on Google Vault from the list of Apps
Now click https://ediscovery.google.com/discovery
A new page opens
Click Retention - top left
Set your retention rules for Mail, Drive, Groups and Hangouts Chat by clicking the pencil icon for each rule
Set all retention rules to Indefinitely as screenshot
All set up. You're admin users will always be able to recover deleted files