Can I expect to have guaranteed break time while working as an English teacher at a private academy?

I work as an English teacher at a private academy in Gangnam. My working hours are from 9 AM to 6 PM, but I work without a break from 9 AM to 5 PM and continue working from 5 PM to 6 PM. I'm curious if it's legally problematic that I don't even have time to go to the restroom. How should I ensure my break time, and how can I respond if the school ignores this? I told the school that this situation is illegal, but they just told me to endure it and have taken no action. Two other teachers and I are experiencing the same issue. I would appreciate advice on how others have dealt with similar situations.