I'm really confused on some issues. I know masturbation and viewing pornography is a sin, but is seeing a girl in a bikini a sin? Also my classmates have girls on their lockscreens on their phones. For instance, this one guy has a girl leaning over showing her butt but she is clothed. She's wearing lingerie. I just need to know is seeing a girl in lingerie or a bikini iniquity? I know porn is, but I'm not sure about bikinis or lingerie. Also, how do I handle nudity in movies?
I believe it is important to label things as God labels them. Nowhere in the Bible is masturbation called a sin. It is a popular teaching that is not backed by Scripture. See: Is masturbation sinful or not?
Pornography is a different matter. It is a form of lust, which is clearly wrong. "For this is the will of God, your sanctification: that you should abstain from sexual immorality; that each of you should know how to possess his own vessel in sanctification and honor, not in passion of lust, like the Gentiles who do not know God" (I Thessalonians 4:3-5). The phrase "passion of lust" translates two Greek words for lust that are used in a row. It refers to depictions of passionate things designed to arouse lust in the viewer. See: A Look at Pornography and Lies Pornography Tells Men.
The question then is what about looking at images where the person is technically clothed, but not well clothed. First, we need to address what the purpose of clothing is and what limits God has placed on clothing, to answer that, see: Proper Attire. After reading this, I want you to think about this: Why would an outfit that covers less that most undergarments be appropriate clothing? Most of us would not be caught dead in our underwear in public, but bikinis cover far less.
The second question for you is: Why do your classmates have images of girls in bikinis on their phone? I would argue that they find such images sexually arousing. It is also why they use images of girls in sexually suggestive poses, even if they are clothed. Consider what Paul said, "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). Images likes these tell others in the world that this person is interested in sex -- a message that Christians should not be giving.
Preferably you should not be going to movies that contains nudity, again it would be showing support for something that goes against Christian teaching. Yet, sometimes you might get caught by a scene that was not mentioned in the reviews. The simple answer is to avert your eyes. Look somewhere else.