You can publish any of your formatives to your school's private library as a Common Assessment. When a colleague grabs this from the library, they won't be able to edit or change the content in any way (they will only be able to assign it). That means that you can be sure every class is taking the same version of the assessment.
To mark your formative as a Common Assessment, simply slide the toggle when you publish to your school's private library:
Common Assessments will be labeled as such in your school's private library. You can also search just for Common Assessments by using the library filters:
Note: anyone who grabs a Common Assessment from the private library is grabbing a LIVE copy. That means that if the original author changes or updates the Common Assessment, those changes will be reflected immediately on the screen of all users who have a copy - including students currently taking the Common Assessment.