The short answer is yes. For several reasons.
First, the very first commandment handed down on Mount Sinai to Moses was "Remember the Sabbath Day, to keep it holy." This means, according to the Catholic Church, to go to Mass on the Lord's Day, which in the Roman Catholic tradition is Sunday, because Jesus rose from the dead on Sunday.
Second, Jesus said in the Great Commandment, "You shall love the Lord your God with all your heart, all your soul, and all your mind." He said this was the first and greatest commandment.
The "all your mind" part also means you are not allowed to have your mind on other things, like video games, conversations with friends, etc., while you are in church. Every thought in your mind should be devoted to God.
BUT, the short answer is not the complete answer. The reason it's not complete is that the question was about "sin," not about "Mass on Sunday."
For this answer, we have to look at a small definition of what sin is. Sin, in a nutshell, is making a decision to say no to God. That's basically the whole thing about what sin is.
When you are 12 or 13, as you are, you don't exactly have control over all the decisions in your life. Adults have some control over what you get to do or not do. I can tell you, adults sometimes don't have control over their behavior either, such as when they are mentally or physically ill. These adults have to give up some control of their lives as well, even though they are much older than you are now.
So, if you take that brief definition of sin into account, can you now imagine how it applies to you missing Mass on the Lord's Day?
Let me tell you a short story. There were three high school seniors who lived in Oak Forest. Just before they graduated, they decided they were going to play a trick on Father Meany. They were going to go to confession but they weren't going to say what they really had done wrong. Instead, the plan was that each one would make up something bad.
So, the first boy went in. "Father Meany, I have sinned. I took some Vodka out of my parents' liquor cabinet and got really drunk," he confessed.
Father Meany paused for a moment. "Hmm," he said. "Well, all young men really need to learn about these things somehow, but next time, just ask, because it was very dangerous for you to drink without supervision like that. Here's your penance. Now go in peace, my son."
The second one went in, and his was a real whopper. "Father, I found some magazines with several pornographic pictures in them, and I looked at them with my friends," he said.
"OK," Father Meany said. "That seems like it's just a normal part of growing up, but I want you to be nicer to people from now on and realize that they are more than what they look like. Here's your penance. Now go in peace, my son."
The third senior went in. He had saved his confession for last, but he thought it was really the mildest of all the sins they had made up. "Father, I didn't go to Mass last week," he confessed.
"WHAT??!! HOW COULD YOU MISS MASS?~!" Father Meany yelled. "Oh, this is really, really bad. I don't know what to do. You have to say a whole Rosary for your penance. Now go in peace."
This story, of course, didn't really happen, and Father Meany would have probably asked a lot more questions to determine just how bad a sin the third young man had committed.
But what would those questions have been? These are the questions you need to ask yourself to determine how bad a sin is, or if some action (or lack of action) on your part is a sin at all.
We know (from Moses and Jesus) that it's a sin not to worship God, give him thanks, show him our love, tell him how great he is, tell him about our close friendship with him, etc., etc., on Sunday. The Old Testament (Moses) and the New Testament (Jesus) are definitely clear on that one.
The question, though, is: Where is the sin? Remember, sinning is making a decision to say no to God (or his commandments). If you were not in church last Sunday, that looks bad, because it looks like you're breaking a commandment that Jesus and therefore God gave you. But it may just look bad. If there was no decision made on your part to turn your back on God, then you have not sinned at all.
Let's take a look at some examples. Suppose your mom asks you, "Do you want me to drive you to Mass this morning?" and you answer, "No, thanks. I'm just going to stay home and play my video game." And then you don't walk to church for Mass later on.
Is that a sin? You bet it is! And not only is it a sin, but the sin itself occurred when you first decided to say no to God, when you first decided not to go to church on Sunday. You had the opportunity, and you turned it down with a simple refusal.
Next, consider this scenario: You ask your mom, "Mom, can you drive me to church this morning?" but your mom says, "Oh honey, the car is broken, and it's too cold out for us to walk. We're going to have to skip Mass this morning."
This one's tricky, because, can you ask your neighbor or your grandma? But OK, if you tried to go to Mass and the circumstances were such that you just absolutely, positively couldn't get there, then you never actually decided to say no to God. There's no sin here, even though it looks the same (you're still not in church) as the first scenario.
Finally, let's consider a third scenario. Suppose you were on your way to church one Sunday, and you saw a car accident on your way. You got out to help the people who were hurt, and then the police came and wanted to talk to you about what you saw. That conversation took so long (because the police were also busy helping people when they got there) that you missed the last Mass. Did you sin? Absolutely not! In fact, many priests would tell you that stopping to help people WAS your Mass. The Lord commands you to keep holy the Sabbath: by "loving your neighbor" who was injured in a car accident, you have certainly made it holy. Instead of doing work for yourself (which would not be good on the Lord's Day), you did the Lord's work by showing kindness to an injured person.
Now, the prophets tell us (e.g., Ezekiel, Hosea, etc.) that we HAVE TO TRY to obey God's commandments and that we won't always be successful. In Jonah, for example, the Lord tells the prophet he HAS to go to a place that he's afraid of. He has to deliver the message and try to get them to turn their lives around. If they didn't listen to Jonah, then at least their destruction at the hands of God wouldn't be on Jonah's conscience. After all, he tried.
It's the same with sin. You have to TRY to do what the Lord commands you to do, in order to be a good friend to him. Whether or not you succeed is not always going to be up to you (it's really up to just God), especially when you can't just get in your car and drive to church for yourself.
The bottom line is, if you try (say, by calling a friend or other relative) when you just can't get to church by normal means, then you are not sinning, even if you can't always make it to church on Sunday. But when your life is more under your own control in a few years, and then, you decide you're not going to Sunday Mass, that's a sin, no doubt about it. The church has several different Masses, including some on Saturday. If you have to be out of town, there's almost not a place in the country that doesn't have a Roman Catholic church. Go to another church, which is still the Lord's House of Prayer.
In short, the Lord doesn't really care about your excuses. He just wants you to visit him every week on his Day. After all, he made that day for you, just like he created everything else in your life that is good. His Spirit is present in the church. Visit him.