I need to mark a large collection of students as 'quarantine' from today out 14 days.

I plan to do this by setting an attendance code of 'Quarantine' on an advisory for those 14 days. [or at least for those school days in the range].

I was just told I could not import any attendance in the future!

I see no reason for this. I can do this manually.

And...as a general rule, ANYTHING I can do manually should also be doable via import.