You may not appreciate my comments - and that would not surprise me. When I was your age, I did not particularly like people older than me telling me “how it is”.
You are a student. I happen to be 71. And in 71 years I have learned at least one or two things, if not more, and some of them were painful to learn.
You do not have enough life experience to be able to make the kind of judgement you want to rush into. I will repeat: you know next to nothing of your friends’ lives. You know nothing of the family dynamics, and I seriously doubt you have enough life experiences to know how to bring up subjects such as this one in a way that does not either come across as judgmental (whether you are or are not), or as sticking your nose into matters which may be different than you are assuming.
You need to keep your nose out of this. It is not your place to correct them - you are not their parent, nor their counselor, nor their confessor. And I seriously doubt you have learned enough of the approach which uses questions instead of statements to make a point.
If one of them, in a serious manner brought the matter up with you - such as a question as to why you go to Mass regularly - you could answer them, without telling them what they are doing is wrong. In other words you need to credit them with a bit more intelligence than you are doing now. and that means they need to bring the subject up - not you.
Fraternal correction is all too often someone coming across, whether they are conscious of it or not, as a self-righteous prig (and if you can’t define that word, look it up). Fraternal correction, as it is used in the New Testament, is about someone who has offended you. These kids are not offending you; they may or may not be offending God. And if you don’t understand that difference, then you most definitely have no business correcting them.
In short, much of what is passed off as “fraternal correction” is being a busy body and sticking your nose in where it does not belong. Not all fraternal correction is wrong, but a highly significant amount of it either should not be done, or definitely not done the way too many people do it.
To answer your last question: it does not concern you; it is between them and their parents and God and their confessor. And yes, it is in the Catechism, but the Catechism doesn’t teach one how to do this. You need to focus on how you need to live according to Christ and the Church, and spend a whole lot more time on that and a whole lot less in looking at what others may be doing or not doing.
And one more clue: I suspect your friends are intelligent enough that they may know what the rules are. Perhaps they may not know, but you are not their teacher: that belongs to their parents, and to catechists. You should keep them in your prayers, and then attend to yourself. Insinuating that they are committing a mortal sin (which they may not be doing) or saying that outright is far more likely to make them mad at you than it is to get them to attend Mass. Believe me, in 71 years I have observed that reaction far too many times. And people just move that much farther away from the Church.