The cassock is used at the Pontifical North American College by those serving Masses, and the choir (both also use the surplice over the cassock at these times). The only other time cassocks are worn are at papal events.
Most seminaries require their seminarians to wear a clerical shirt at most times. If the PNAC is still doing what they were last I checked, seminarians wear clerical shirts in the morning, and change into “business-casual” for afternoons at the college (after classes).
Having seen you post previously on the vocation forms, I want to offer the following advice: Don’t get too hung up on the cassock. I like my cassock. I prefer the look of a cassock to a clerical shirt. But as a seminarian, and even a young priest, you can’t expect to be able to use it all the time (or even often).
There are many priests of a certain age who don’t like the cassock, because they relate it with “clericalism,” or “old-fashion-ness.” Neither of these are necessarily true of course, but many priests have been formed in such a way that this is what they have been told for decades.
If you’re accepted as a seminarian, keep your head low, do what you’re told. If (God willing) you’re ordained a priest, keep your head low still. If you build a rapport with the bishop and priests of the diocese as a priest who’s a good, hard-working associate pastor, after a couple years when you would have your own parish, these things will be given a bit more slack, and you can slowly work into showing more of your personal “style.”