Forced overtime during my examinations?

I am working part-time for a betting company (25 hrs per week). I also studying at home. I do not want to work overtime during my exams as it affects on my study or sometime few other weeks coz of personal reasons.. My contract is saying "where business needs dictate, Employer will be required to work additional hours/days beyond their normal commitment". My DOM said I have to do it and there is no way out. Is it correct?