question

question

I work for job where the hours of operation are 8 am 8 pm cst.  We dont have a union and we bid for our shift.  My hours are 11:15 am to 8 pm cst.  Now we can’t leave until email is sent out for us to leave.  I go to my supervisor to let her know I Need to leave exactly at 8 pm, and she said if I leave without the email being sent it is a occurrence.  Is this illegal.  I work in state of TEXAS…… I thought the law labor law said unless there is agreement.  Companies have to follow agreement Need help