[ … ] I can't get it to work...what am I doing wrong?
How do you expect an answer to that question without showing your script? But …
[ … ] I've created variables in a script for each field [ … ]
… have you considered simply importing the student record into the alumni table?
Another question I would ask is have you considered having one "people" table, with some sort of status field that would indicate if they are a student or alumni (or other possibilities)?
But as for troubleshooting your issue, without seeing your script, it's hard to say.
Do you have FileMaker Advanced? Using the Script Debugger and Data Viewer to step through your script can often find your flaws pretty quickly.