I hear you. I think you have also have to build the high school on the site of Copperdale High, too. I know some went ahead and built new buildings, but in placing them they didn't function like the high school is supposed to. They have really shortened our arms on this one.
Do an Internet search on Sims 4 mods. There's one called Choose your High School Classmates, by Rex and another one that is supposed to work in conjunction with that one that allows you to have up to eighteen classmates. I'm still trying to see if they work. I've had to restart my save more than a couple of times, so since I added the second one, which allows for eighteen students, I haven't yet had the chance to see if I can get the triplets into school with my active Sims. Best of luck.