Removing active families from your import file
Log into your account, and go to Account Settings / Export
Select Students for Administration. This will export one row per student which you can use to match up to your spreadsheet. The best way we've found to do this is to create a new column with the following formula: Last_Name + student name + grade. You would do this for both the export of the live data and your import file.
You can then match families up (using Excel’s vLookup() function), based on the new column, and remove any students already in your account. This should leave you with just the new students, even if they have siblings in the school. For these incoming siblings, our import will simply add them to the existing family as long as at least one of the parents's emails match as between the import file and the active data.