Most retailers don't have the concept of unpaid leave for salesclerks. You could certainly take a look at your particular employer's employee handbook and see if your employer offers this. But I wouldn't have high expectations.
If you accrue and maintain seniority that is beneficial (i.e. it allows you to get preferred shifts), it would be pretty common to drop your hours down to some minimal threshold. There are lots of people that work a shift every week or a shift every couple weeks while they are transitioning to a new career just to stay active in the payroll system. That generally makes it relatively easy to ramp your hours back up if you get to the end of your internship without a permanent offer on the table.
If you don't want to deal with juggling retail shifts and your internship, most retailers in the current environment are hiring pretty continually. So if you give appropriate notice when you resign, it's pretty likely that they'll happily welcome you back if you apply when your internship is coming to a close. Particularly when you're leaving for an educational opportunity rather than because you disliked the job.