If your school has become an academy, all you need to do in Arbor is change your school's opening date.
You can see how to do this here: Changing your opening date
This will then change your school information in the background. In the census, the data considered and output will only be from your new opening date onwards.
On student profiles, you'll then automatically have two records in the Educational History section for the enrolment into the old school name prior to the academisation date, and the new academy following that date.