How to do this gracefully?

I currently work 30hr/week at a bank in addition to working going to school full time but just accepted an internship at Fortune 500 company in a role that directly correlates with my major. I obviously will need to cut down on hours at the bank so I can do this internship, but am nervous about going into my bosses office and telling her I’ll only be able to work max 15 hours a week. This internship also pays 3.50/hr more than my current job does, so I’m also considering just putting in my two weeks to focus on the internship. What should I do, and how do I do it gracefully?

TL;DR accepted an internship and either need to cut hours or put in two weeks at work, unsure of what/how to do it

edited for clarity

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