1ºMortal Sin has a subtle definition best given by a priest in person.
2ºCheating on exams is certainly a sin against Truth!
3ºYour need to “cheat” was an effect of the prior sin of “Sloth”: You needed to cheat because you had been lazy before. Start being “Diligent” about your work and you’ll have no need to cheat.
4ºThere is a difference between “mortal sin”, “venial sin” and “capital sin”. Start your research there.
5ºThere is a Virtue opposed to each Sin. What Virtue did you lack that led you into Sin?