Why won't my phone stay in priority mode?

[AC Question]

I have set a downtime from 6:00 am-4:00 pm so that my phone doesn't go off at school. Well, after it went off while I was at school, I began checking each morning. When I wake up it's on "none-indefinitely" but around 7:00 am, it goes back to all ringtones/ notifications being on. I always put it back on priority mode, but each morning I have to repeat the cycle.