Use your Lead Properties filters to find those leads ready to be exported to your SIS.
Firstly, from your leads page select Advanced Filter. Then depending at what point in your enrolment journey you export to your SIS select either Forms>Enrolment Application or Enrolment Acceptance > Contact Filled out the Form.
Then add another filter Lead Properties > Last Export Occurred At (this is the very last option in your dropdown list) > Is Empty. This filter should look like the below.
Remember, this depends on your import process. You may only import confirmed enrolments once they have completed all of the requirements of your process so your filter may look like the below, by selecting both Enrolment Application and Enrolment Acceptance or potentially add on the stage they need to be in at point of import.
If you save this as a Dynamic List, this will be live and updating, so once you process the export from Funnel those leads will be removed automatically from this list as they would then have a Last Export Occurred at date.