I've written MG and YA and I always struggle with slang because there are so many different types of slang.
My kid who is an advanced student doesn't use slang when she talks to some of her friends, but uses it a lot when talking to others.
Also, there is different types of slang. There's street slang, suburb spin off street slang, preppy girl slang, cultural slang....
What to do? What do you do?
My pub told me to slim out on the slang because it's too many different versions of it, and it doesn't fit in with the character profiles of some of the books I've written. Yet some reviewers mention they would prefer more slang.
Avoid it. When you're using slang that doesn't come naturally to you, it's very apparent. IMO the first book in the "Marked" series really shows this (the speech rhythms sound more natural with subsequent books).
Plus, it really dates the novel. Imagine if JK Rowling used slang in Harry Potter. If Ron said stuff like, "It's da bomb!" and "It's wicked awesome!" back when that was big (HP started in the 90s, right?), do you think the stories would have remained so timeless for each new group of kids who became old enough to read them? They probably would have been distanced from the story the instant they saw the outdated slang.