What is the ruling on a student intentionally giving wrong answers to classmates who are trying to cheat from his paper, thereby causing them to fail, and is he required to do anything to avoid falling into this mistake in the future?

You were correct in not participating with your colleagues in cheating, for it is forbidden. However, what you did by putting wrong answers on the exam paper is impermissible, because it is treating one evil with another. It would have sufficed for you to strive to hide your paper from them. You should advise them gently, remind them of Allah, and encourage them to strive [in their studies].