This has to be Double-Dare to Be Scared by Robert D. San Souci.
Published in 2004, 170 pages, with a black & white cover featuring a creepy-looking kid, and black & white illustrations inside.
The book can be borrowed from the Internet Archive.
From the story "Campfire Tale":
It begins with a kid, his dad, and a co-worker (not uncle).
A few days before starting fourth grade, Michael Johnstone went camping with his father, Dave, for the very first time. He had hoped it would be just he and his father making the two-day trek to Paiute Lake, but his father had invited a coworker, Larry Kinross, to come along.
Once there, by the time they finish dinner, the woods are dark and Larry has a story to tell.
Larry took a big gulp of coffee, then lowered his voice as though he were sharing a secret and said to Michael, “Your father didn’t want me to tell you about The Stalker.”
In spite of himself, Michael asked, “What's that?”
“No one knows for sure if it’s some crazy person or some kind of monster like Big Foot or what. But he—or it—lives in these woods. He sneaks up behind hikers and campers and literally frightens them to death. Folks are found with their mouths frozen into an ‘O’ from pure terror.
Later, after going to bed, Michael wakes up needing to use the bathroom. His father and Larry are still sitting by the fire, talking. Michael takes a flashlight and heads into the woods to do his business, but manages to get lost on the way back. He finally finds his way back and sees his father and Larry still sitting by the fire, apparently asleep.
Michael decided to play a trick on them.
Quietly, he crept up behind the two dozing figures. When he was an arm’s length away, he suddenly reached out, grabbed their shoulders, and yelled, “BOO!”
Both men pitched backward off the log.
In the fading glow of the dying campfire, Michael could see the double ‘O’s of their fright-frozen mouths.
Then a twig snapped behind him.
From the story "Class Cootie":
It begins like this.
Anthony was slow to understand a lot of things: multiplication tables and prepositions and how to spell principal. But he knew that everyone at Charlotte Forten Elementary School—the teachers, the staff, and the 823 students—thought him a fool and laughed at him.
Anthony lives with his grandmother who came from somewhere in Eastern Europe. The other kids called his grandmother a “witchy woman.”
During a food fight at school, with most of the things being thrown at Anthony, he runs out of the school toward home, gets hit by a car, and dies.
The school holds a memorial service attended by Anthony's grandmother who lets everyone know she holds them all responsible for his death because of the way they treated Anthony. She leaves and the students return to their classes.
Later, everyone notices something outside and they crowd around the window to see.
The sky was raining insects—massive, brightly colored bugs with black, yellow, green, and red bodies. When they hit the ground, they scuttled forward on pink legs. Green antennae moved rapidly detecting sounds; glittering yellow eyes darted back and forth, seeking movement. These shiny, impossible creatures had glistening metal claws; and the ends of their eagerly unrolled feeding tubes, lashing back and forth, had wicked-looking tiny blades.
The insects swarm across the parking lot and cover the school windows, piling on more and more.
Then the first windowpane exploded inward from its frame, the shards of glass scattering across the floor. There was the din of gnawing, gnashing, clicking, buzzing everywhere, as the brightly colored creatures poured through the opening. But the sounds they made were quickly drowned out by the shouts and screams that filled both floors of the darkened school.
From the story "Grey":
It's not about finding a UFO, but about finding an alien called Grey.
“Grey comes from another place,” Alec went on. “He came as planned, but something happened to the equipment that brought him here and was supposed to help him with his work.”