The harm is that there's a bias in their marking standards and the school will be seriously disrupted by students/parents questioning this. Sure you can draw an analogy between this and an employer screwing one of his employees, but I feel the difference here would be that.
- Kids have to go to school and the majority are basically stuck at their one public school.
- It's the employers decision to have such ruccus in his business... For a better analogy we could think of a business run by shareholders, many of whom are employees and families of the employee's. Would those shareholders want the sort of situation that would occur with that employer screwing around with an employee? I doubt it.