Cheating in relationships is wrong because it hurts the person you are cheating on. Cheating on a test is wrong because you will not learn that way and it is not fair to the people that did learn it.
Notice I wrote wrong and not sin.
Sin only exists in the minds of believers and will change from year to year and from religion to religion. For some religion it is a sin to eat pork, for some religion it is sin to listen to Heavy Metal music, for some religion America is a sin,etc etc etc.