You can add a Tag to all students in your results list when you run a Student Database Segment:

Step 1 - Find the students you want to tag

Run your segment and view the results. Click on the Options menu to add a Tag to these students:

Step 2 - Select an existing tag or create a new one

When you click 'save' you get a confirmation message telling you the tag has been added to all of those students.

Step 3 - view your tags

On each student, your new tag now appears. They are ready to be used in a fresh segment or in a student search.