Let me first congratulate you for being so articulate and investigative at such an early age. There are rewards in life for asking intelligent questions.
I am not a medical professional, but as a scientist and as a healthy middle aged man with kids roughly your age, I can chip in here. Young people have an affinity towards high-energy, low fiber foods like cheeseburgers and fries. No matter how much I counsel my adolescent sons on this, they tend to gravitate towards this junk. I used to be even worse...but I was uninformed.
You should read health-oriented publications like "Nutrition Action." Good habits begun at an early age pay big dividends over a lifetime.
That your stools are "large" tips me to think you're constipated. This is symptomatic of improper diet. Yeah, that's right, it's dietary... and a common issue that is not a topic of conversation among young people. So people can grow up and live in ignorance for decades...like I did.
Believe it or not, I figured this out for the first time 20 years ago when I was re-habbing my house, which forced me to eat in restaurants a lot. There was nothing uncommon in THAT for me, but by coincidence I frequented Burger King on this occasion. I noticed incredible constipation resulting directly from my diet.
There are other mitigating circumstances which can obstruct such direct deduction. But after connecting the dots, I realized my diet as a young person had been improper in its lack of fiber.
With my suspicions up, ever after since the "Constipation King" episode, I began experimenting with fiber breakfasts, even over doing it with resultant gas. Interestingly, constipation arises when we take extended trips away from home...and our familiar toilets, indicating a signficant psychological factor. This is particularly evident in one of my children, but not the others. One significant point I deduced in my own physiological case was that on a business trip where I carried high fiber breakfast food with me, I became constipated nevertheless. The conclusion: I was not drinking enough water. Yes, water is the other half of the "equation" in maintaining bowel regularity.
I am speaking to you as if you're my son at this point. So perhaps you might speak with your own parents, too. Meanwhile, don't become upset. You have most likely stumbled onto a mundane problem that is tolerated by countless uninformed people...especially young people like yourself.
Learn about diet. You can avert obesity and constipation, not to mention serious degenerative conditions, etc. Stay away from junk food, which is caked with fat and sodium. Learn about fiber ... and water.
What do you eat for breakfast? You need a good breakfast, and I suggest bran-based cereals. That's the place to start. Drink plenty of water and get some exercise.
I'd stay away from extreme measures like harsh chemicals to purge your bowel. Consider a suppository at times of stress.
If you push too hard while on the toilet, you can foment hemorrhoids...though not necessarily permanent change. It's also a bad habit that can kill you when you're very old and frail. If your waste doesn't pass easily, you should first suspect diet. It's habitually overlooked in young people...though I have only immediate family members for statistical samples. Nevertheless, I induce this problem must be widespread, given the ubiquitous consumption of burgers and fries.