What's the Name of That Book??? discussion

read around 2017-2019, a new book
the book is set in the timeframe of about 6-8 years, as a young boy finds himself in a magical summer camp away from the modern world

I'm honestly not quite sure that it's exclusively a summer thing but whatever

the protagonist is a loner and meets a blond kid that everyone seems to get along well enough with (except for him). the years progress and the main trio (2 guys, 1 girl) discover more and more about the world. the two boys have a rivalry going on, and are competing for the girl's affection for the first few years. In the camp, the kids learn history and such, and roles to fit into the magical world. blond kid is a warrior of sorts and the protagonist is a diplomat.

they're introduced to this world as children bc once they become adults or smth they become stuck in one world or another and have to choose. the protagonist's mother may have been important in the past idk.

one of the overarching conflicts is the divide between fantasy races (?) the diplomat thing is central to the plot

there's quite a lot of plot centered around the personal lives and interactions between the campers, most notably in the latest years of the book, the two guys from before start dating

the protagonist ends up staying in the magical world, but not before smuggling over a lifetime supply of pens and markers

the camp is in the united states, the magical world is hidden from regular humans by the normal people just,, not seeing it? there's a wall kinda thing separating them

some of the adults that chose to stay in the mundane world work with each other to bring some kids they suspect would be able to see it to the wall where the worlds either cross over or are hidden

the whole world is big though, so its probably a crossover in realms thing

In Other Lands, by Sarah Rees Brennan?

"The Borderlands aren’t like anywhere else. Don’t try to smuggle a phone or any other piece of technology over the wall that marks the Border — unless you enjoy a fireworks display in your backpack. (Ballpoint pens are okay.) There are elves, harpies, and — best of all as far as Elliot is concerned — mermaids. Elliot? Who’s Elliot? Elliot is thirteen years old. He’s smart and just a tiny bit obnoxious. Sometimes more than a tiny bit. When his class goes on a field trip and he can see a wall that no one else can see, he is given the chance to go to school in the Borderlands. It turns out that on the other side of the wall, classes involve a lot more weaponry and fitness training and fewer mermaids than he expected. On the other hand, there’s Serene-Heart-in-the-Chaos-of-Battle, an elven warrior who is more beautiful than anyone Elliot has ever seen, and then there’s her human friend Luke: sunny, blond, and annoyingly likeable. There are lots of interesting books. There’s even the chance Elliot might be able to change the world."

This is it thank you! :))))In Other Lands