Can a school delete one constituent from a previous their data import that was made using the School Level Data Importer(SLDI)?

Yes -

You can delete constituents by visiting the Data Management section of your school dashboard (Data Import > Data Management). In the "Delete Imported Data" panel, select "Constituents" as the type of data you want to delete and

Upload a CSV with the constituent ID(s) to be deleted, then click Upload IDs to Delete.

Note: Constituents used in GC Gift Officer cannot be deleted using this tool.