Dear Economist: Help! How do I keep my students in class?

I teach special education at an urban public high school. Every period, as many as half my students ask to go to the bathroom. I am pretty confident that most of them are just looking for an excuse to leave the classroom, but going to the bathroom is a right and I don’t want to run my class like a prison.
Beside making my content more engaging, how do I allow students to go to the bathroom in a way that’s equitable and limits the numbers outside the classroom in a given period? Three stipulations: grades are off-limits; I also don’t have much money, so I’m reluctant to offer prizes; and I’d like this system to have as little to do with me as possible. I’m a teacher, not a gatekeeper.
Anon, Washington, DC

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