Examples
A school refuses to let a gay pupil become a prefect because of his sexual orientation. This would be direct discrimination.
A selective school imposes a higher standard for admission to applicants from an ethnic minority background, or to girls. This would be direct discrimination.
A pupil alleges, in good faith, that his school has discriminated against him because of his religion (for example claiming that he is given worse marks than other pupils because he is Jewish), so the school punishes him by making him do a detention. This would be victimisation.
A teacher ridicules a particular pupil in class because of his disability, or makes comments which have the result of making the girls in the class feel embarrassed and humiliated. This would be harassment.