Was I wrong to inform the teacher about the theft of the exam paper and its distribution among students after I refused to take it, especially after I was subjected to ostracism and threats from my classmates?

The action you took is a legitimate act of scientific and social reform. Forbidding people from deception and fraud, and enjoining them to be just in their dealings, are among the duties of messengers and their followers, as Allah Almighty says: ﴿And let there be [arising] from you a nation inviting to [all that is] good, enjoining what is right and forbidding what is wrong, and those are the successful.﴾

Your friend's claim that this matter does not concern you is false, because the spread of such behaviors is corruption that harms the scientific community.

As for the students' animosity towards you, do not be saddened by it, for it is due to your upholding the truth and your advising them. You must treat them kindly even if they show hostility towards you, for the Prophet, peace and blessings be upon him, said: "You will not enter Paradise until you believe, and you will not believe until you love one another. Shall I not tell you of something which, if you do it, you will love one another? Spread peace among yourselves."

Regarding informing the administration or professors about students' bad behavior, a balance must be struck between benefits and harms. Sometimes concealment is more beneficial, and sometimes informing is more beneficial if the offender is audacious in committing evil deeds, and the harm of his sin affects everyone.

You should weigh the benefits and harms if this or similar incidents occur again, and consider the importance of the exam. Some exams are important, their grades are recorded, and students are evaluated based on them, while others are of less importance.