I don't know how the game decides who to bring in for the live school day, but I'm pretty sure it doesn't generate new sims just as high school students. It definitely brings in teens from everywhere. I have seen Cassandra Goth, Morgan Fyres, Sophia Bjergssen, Wolfgang Munch, Duane Talla, Luna and Hugo Villareal, Yuki Behr, and probably plenty of other premades as classmates. One of my games even featured Jacob Volkov, stuck in raging werewolf form all the time.
You definitely do get random generated classmates, but I don't think they were generated just to populate the high school. It's more that they are in the game, and they're teens, so they show up.
You will probably start noticing that not all of your classmates are the same every day. And if you attend the football and cheerleading events, you'll see teens there that you have never seen at school.
I don't know of any way to make teens that you would like to run into, show up at school. There's a nice mod idea, for anyone looking for something to make!