Does cracking your knuckles cause arthritis? I do not have the definitive answer, as not many people have studied this frequently asked question. Your mom and your teacher are probably both right. To my knowledge, it has never been scientifically proven that knuckle-cracking causes arthritis.
Many people argue that cracking knuckles causes our joints to move in ways they were not designed, putting extra stress on cartilage or ligaments. One study reported a higher incidence of hand swelling and lower grip strength in people who admitted to being habitual knuckle crackers.
So, should you crack your knuckles? Hard to say. It may make your knuckles bigger or it may cause your hand to seem weaker. It might cause arthritis or it might not. Either way, it definitely annoys many people. Although it may feel good, it does not clearly do anything good, so it's probably a habit worth breaking.