Yes, nobody is counting the number of teenagers and creating that many jobs, that's not a thing that happens in a market economy. The actual mechanics are that a certain amount of unskilled labor exists, so companies adjust how their businesses operate to take advantage of it. If labor is expensive, businesses find a way to reduce labor needs (e.g. automation), and if it's cheap, they create jobs.

So, if we increase the minimum wage, businesses will hire fewer teens because they're too expensive for the quality of labor and inflexibility of schedules. If we decrease the minimum wage, they may find a way to use more of that cheaper labor.

open during school hours

Yeah, that's one of their busiest times, so they'll make sure their labor needs are met. Maybe they'll pay more, or use college students who have more flexible schedules. Teens tend to get less valuable shifts, like late nights, and that's for a reason.

If labor is too expensive, they'll also probably just close earlier because the labor costs aren't worth the minimal business they'd get.

If we instead use something like a Negative Income Tax or Universal Basic Income, it won't matter if wages go down because people will have enough to live on. And if we only provide NIT to citizens and permanent residents, we won't have as much competition at the low end and can reserve those jobs for our teenagers. So a teen could make $5/hr and be happy because they don't need to pay rent, and a college student could make $5/hr and receive $10/hr or whatever as NIT and be happy because they can afford rent and tuition. We don't need a $15 minimum wage in that scenario.