Democratic Primaries

...the percentage of under-45 voters who are in college is small. Which I mention because the stats I've been referencing show that Sanders does much better than Biden with voters under 45, not merely those 18-21.

And BTW, those 18-21 year olds who go away to school and come back to their parents' homes when school is closed are probably a minority even among 18-21 year olds. Many 18-21 year olds don't go to college at all. Many who DO go to college go to local community colleges or commuter schools. And even many who have gone away actually establish full-time (non-dorm) residence in their new city, especially in their later years.

Spring break could also have some of the opposite effect of what you're talking about (or what it usually would). Kids who would normally go away for spring break may now be staying put because so many travel events are being cancelled.

So... yeah, you've brought up another interesting factor... and another bunch of unknowns.