I'm 15 years of age. A few days ago, in my school, we had a reenactment of a battle. When one guy was "dying," he was moaning and and made sex sounds. Everyone started laughing. Even I couldn't help but laugh. Is it a sin to laugh at things like that? Sometimes when my friends make jokes like that, I can't help but laugh. Is this a sin?
It is typical to find the unexpected humorous, but that doesn't mean all humor is good. "But fornication and all uncleanness or covetousness, let it not even be named among you, as is fitting for saints; neither filthiness, nor foolish talking, nor coarse jesting, which are not fitting, but rather giving of thanks. For this you know, that no fornicator, unclean person, nor covetous man, who is an idolater, has any inheritance in the kingdom of Christ and God. Let no one deceive you with empty words, for because of these things the wrath of God comes upon the sons of disobedience. Therefore do not be partakers with them" (Ephesians 5:3-7).
What your classmate did was inappropriate. It caught you off guard, even though you know that such sexual humor is inappropriate for Christians. Pray to God that you did not leave someone with the wrong impression during that moment and then move on with your life.