I am pretty sure you have to make the request. How else are they supposed to know you want the deferment? Also, I am pretty sure loan origination and servicing are separate, so they might not even know that you are starting school. Finally, you'd be eligible for a deferment even if you aren't borrowing for med school, and, if you aren't taking new loans, they'd certainly have no way to know you are going back to school.
This is good to know! Anyone who is lucky enough to receive a no-loan package should probably call their servicer just to be safe, since there probably wouldn't be anything to trigger it if you don't take more federal loans.
If your undergrad or grad school loans were all federal then re-enrolling in school triggers an automatic deferral, because the financial aid department at your medical school has to submit to the federal gov that you’re once again in school. If you had taken out private loans they might not be linked, so there would be a separated process you would have to go through.
Federal student loans is one of the easier government programs to navigate, until it comes time to refinance.