ditto what michael k said plus: going from Panera to Starbucks is EXACTLY the SAME job for EXACTLY the SAME money. If you are bored at Panera, you will not be helping yourself. All you are doing is trading in a boss who knows you and is willing to deal with your scheduling needs for one who might not.
you're a high school senior applying for college, that means you'll only be there for another year anyway, and it's only one day a week. shouldn't be that hard to just put up with it. another thing to keep in mind -- if your boss is willing to keep you around one day a week that means you have their respect.
remember -- with this job scarcity, keeping someone around with such limited availability job means taking shifts away from someone who does need the job and wants to work more. there are only so many shifts. if they are doing that for you, you must be worth something to them. it also means you are far less likely to get hired somewhere else since there are people lined up around the block who really DO need to work and are willing to offer better availability.
also, when you get to college, if you need a job, you have an in: a good reference, no need to be trained, probably still on the books. there are paneras all over the place.
i would stay at panera. you have a lot more to lose if you quit than you would gain by looking for another job. it isn't the last job you'll ever have -- but it could be a good bridge to help you get to the next part of your life.