After attending and then teaching in a Christian high school, I started to realize how often the buzzword "gossip" comes up. It is almost as if it is a "don't do" rule that is constantly used to inflict guilt, but the meaning the rule is often omitted. What is the real issue, when it comes to gossipping? Other posters have basically stated it: disrespect.
I suppose, if you wanted advice, you could change your thinking from one that says, "Am I gossipping?" to one that says, "Am I being disrespectful?"
To be honest, this is a pretty good lesson to learn early in life. You will find that the temptation to "gossip" or complain or badmouth people and situations in the professional world is just as prevalent (if not more so) as it is in high school. Sad. On the other hand, the consequences of saying the wrong thing to the wrong person, in the professional world, are often far more impactful than they were in high school. Your awareness of this issue is great, at your age. Now raise it up a level and consider what "to do" rather than what "not to do."
I would suggest that which makes gossip bad is the intent or the result. Casual discussion about people you know and their experiences is not a problem unless your intent is to be mean, malicious, or downright hateful. It is also a problem if you do not intend these things, but the gossip ends up hurting someone.
I have always told my own children and students that they should not say something about someone who is not present that they would not say to that person face to face. I feel like if we are able to follow that rule we will avoid gossip for the most part. I also have no problem telling others that I really don't want to be a part of that type of conservation and moving on to another subject.
Perhaps we need to clarify that it is the intention behind the gossip that constitutes the right or wrong of what is said. Thus, in concurrence with the above post, your conscience is what makes you feel badly. After all, it is human to talk about friends and family. As Crooks from John Steinbeck's Of Mice and Men remarks, a person needs someone else, or "He got nothing to measure by."
Talking about other people isn't bad unless you are tearing them down in order to make yourself feel better. It's OK to express concern over a friend's weight gain if you are concerned for his or her physical or emotional health. It is not OK to comment on how awful they look and then laugh about it. If you feel bad about something you have said, that is probably a good indictation that you have crossed the line -- you conscience knows the difference.
I have never understood why people think gossip is such a bad thing. Gossip is simply talking about what is going on with people that you know. There is no reason to feel bad about that, in my opinion.
Now, if someone tells you something in confidence, as a secret, it is not a good thing to go and tell other people. But that is not gossip, that's worse. As long as you are not telling things that were told to you as secrets, there's nothing wrong. We are naturally interested in other people's lives. Comparing their lives to ours is one way of figuring out who we are (what makes us distinctive). It is a way of thinking about different kinds of behavior and finding out what our peer group thinks of those things. It serves a useful social purpose.
So anyway, if you're not going out and telling secrets, why feel bad?
Nowadays, gossip is considered a bad word, that describes a bad habit. In fact, there are several advantages of gossiping.
For instance, the act of sharing secrets with your friends makes you a very strong group, that has a distinctive identity. If you are feeling bad about what you've shared with your friends perhaps, now, you are not agree with the idea, but you've done it and that's it.
If your friendship is really strong, don't be afraid to expose your opinions about your topics, as long as they are constructive and not distructive, because, yes, excessive gossiping could become nossy or even agressive.
So, talking or gossiping, is a good way to learn how to act or not to act in a certain situation.