Days are a poor measure here. Hours matter as well and teachers are salaried. They don't earn OT, but they often work more than their contracted hours without compensation. In that study the average was 12 uncompensated hours per week, assuming an 8 hour day over the 36 weeks of a school year that's more than 50 uncompensated days of labor.
I object to the claim (in my quoted section above) that teachers only work 180 days a year. They work more than that, either in the excess hours beyond their contracted hours, in the time they take without direct compensation to maintain certifications or earn graduate degrees, or the time they spend working a second job or summer school or running a club just to make ends meet.