The notion of closing a whole classroom due to a single positive case is ridiculous and counterproductive. We're all going to be exposed occasionally regardless of what protective measures we take. Fortunately CDC data clearly shows that on average COVID-19 is less dangerous to children than other common viruses like RSV.
And no, closing classrooms isn't necessary to protect teachers. They've been vaccinated for months now, and are free to wear PPE if they choose.