Can a teacher be hired and then fired a week later bc there are "no positions available"?

Someone I know is a preschool teacher at head start. Shes been working there for 30 something years. Last year she was told that she needed to have her bachelors by spring of next year. She was 2 classes away and enrolled in one of those classes and the other she had to take next semester. a few days before going back to work she was told if she didnt have her bachelors by october (the MIDDLE of the semester!) she was going to have to be an assistant. When she arrived for work they told her she was going to be a teacher, even though she was fine with being an assistant.They even gave her a hire letter signed by 4 people including the CEO. A few days later they fired her claiming they didnt have any positions available, but yet another teacher who has less experience and is farther behind in her schooling was offered a posistion as an assistant. Could there be a case here? Or is what they did legal? I'd REALLY appreciate any answers and explanations!