Yes, you may be fired for this. Your responsibility is to work whatever hours or shifts your employer tells you to, and to stay as late as they require: whether it is fair or not is relevant, since the law does not enforce (or care about) fairness at work. Unless you have a written employment contract setting your hours, you have to work whenever your employer tells you too, and so may be fired for not working when instructed to. Remember: except when there is a written employment contract, all employment in this country is "employment at will"--you only have a job so long as the employer chooses to employ you, and your employer, not you, controls and defines your job.