No. You might mention it to the pharmacist (my first advice I edited out mentioned it) if the pharmacist has a personal connection to you / would be sad to see you go. That way it's not out of left field when you quit.
Alternately, after a couple days of school, ask to have a formal meeting with the PIC and explain that you'd really truly love to keep the job, but as he knows (since he wrote your LoR), school must be your top priority. If they don't change it on your next schedule, you will be forced to quit to pursue school - nothing personal, and again, if something part time comes up, you'd be glad to have it. But you understand if they need full-time employees - you'd rather leave so they can find an employee that will be able to meet their needs than not be able to keep up your obligation.
It depends on who's pulling the strings (do you need to talk to someone above your pharmacist?) and how that person deals with confrontation. My current boss (not pharmacy) seems to prefer to know early so he can try to accommodate and would prefer to schedule a short sit-down (we're customer facing, so it's best if we can shut the door for a minute). I have had bosses that would prefer I just come talk to them briefly in the afternoon when they're less busy than schedule something. I have had some that can just take the hint from conversation, but those are few & far between. Think about what style your boss would prefer.