Throughout my high school experience as I am 16 years old I have never found it easy to have friends.

High school from a social aspect is an odd experience. You're clumped together and expected to form friendships that are thought to last a lifetime (SPOILER ALERT: most don't).

...it is that children seem to believe in cliques and that people that they don't know they should hate.

Cliques don't go away, they're just no longer seen the same way. These people become the friends you continue to hang out with. Consider going to a restuarant with 9 friends (I know, you're 16, but just play along). No one expects you to befriend everyone else at the restuarant, you just talk with your friends. It's because of that aforementioned expectation to be friends with everyone that cliques can cause difficulty.

Part of the reason I believe is because we categorize people and we can't respectfully disagree on topics of interest.

Again, this isn't just high school. Have a good look at social media or the various news outlets (especially those with comment sections). It has become an expectation for people to think the same and those that disagree are considered intolerent. It's because of this it's so common to hear of topics to avoid in the workplace so we don't offend anyone. That right there is one of my biggest issues: fear of offending people. I get it, you don't want to go out of your way to purposefully offend someone, but to avoid saying or doing certain things just in case is ridiculous! Growing up we were taught, "sticks and stones may break my bones, but words will never hurt me." That phrase has become irrelevant. Too many parents now are avoiding activities because they're child might get hurt, dodgeball was banned because of potential injury and now we're teaching our kids to disagree is wrong and to conform with the social norm.

I believe that people and teenagers today are not nice because they have the mindset that they have to look good to feel good. In all honesty I feel good just by being a nice guy.

Tribalism as aneeded aspect of human psychology and sociology dates back to before civilization. Cliques are simply a subset of the tribal mentality, and in your case develop around the time that people begin self segregating based on group identity. It's shifty and obnoxious, but to be expected to be honest, as it continues outside of school (albeit in a less arbitrary way).

The problem with alternative education (spent much of my young life in a very small private school) is that it doesn't prepare you to deal with this kind of tribalism once you are out of school. I would argue that the greatest lesson you get from public school is how to deal with socialization and navigating group identity.

Because roughly half of them have an erection fairly frequently ;)

I think just about everyone who goes through high school, be it past or present, has had a difficult time socially. You're just starting to find yourself. You're growing as a person, you aren't as dependent on your parents and you're starting to take those first steps into the adult world. And you're doing it without really knowing who you are and what you're capable of. So it can seem hard. Would I want to go back to high school? Hell no. But as you grow you'll find your way.