Have your boss put down in writing a job description and hours expected. Anytime you or any other employee does not live up to it, it is grounds for dismissal. At the same time, unless you have a job description with a list of expected duties... you have no way to prove you went above and beyond and others bailed. It will be hard to play favorites if everyone has it in writing what is expected. Because if they walk out...they can get fired.
start documenting things....date, time, what was said, witnesses. get a job description. believe me, your boss may not want to be accountable for stuff....get things in writing. start a paper trail. protect yourself. you can sue for wrongful termination. whistleblowing. you will need to be tough when the going gets tough. and it will get tough thats why you start a paper trail so you can sue. arm yourself with a good defense. contact your states eeoc. you dont have to take crap.