|If I'm interpreting your plot correctly, your twist here, and it's a fun one, is that the person the narrator assumed was a guy all these years is actually a girl. With your choice of words and phrases here (the fake smile, etc.), you do a great job of conveying the narrator's continuing annoyance with Jason, which makes what finally happens logical and believable.
At first I thought the other kid's name was Jason, which would have fit the story line, since Jason could be a girl's name. But the last sentence suggests something else, that the narrator had simply been mistaken about the kid's name all along. If that's true, I was left wondering why "Jason" wouldn't have said something to her about it at some point during the five years they've been acquainted. In any case, I was left a little confused about what's going on here.
I love how you kept the story moving right along with the dialogue and tone, a really enjoyable read.
As it happens, I wrote something a little like this a few years ago. I called it "Out," and here it is, from my Writing.com portfolio, in case you'd like to take a peek:
I wanted to keep playing. Jeez, I was two for three at the plate already, blocked about a million wild pitches from that crazy Kulisek, who shouldn’t even be allowed on the mound, he’s going to kill somebody, I swear. But my mom said to be home by three, write the thank-you note to my grandmother, it’s so overdue it’s embarrassing, blah blah blah. Don’t you get a year or something? I heard my mom and dad say once that you get a year for wedding presents. Why not the same for birthdays?
I nail some idiot at the plate to end the inning. Trying to score from third when he thought one of Kulisek’s fancy curve balls was heading for Mars. Sorry, pal. Holding the ball, I plant my bare hand on his trunk as he starts to slide under me. Close, but out. Out, out, out.
“I’ve gotta go,” I tell Kulisek as we head off the field.
“What’re you talkin’ about? Go where?”
“My mom wants me home by three.”
Kulisek is pissed. “You can’t go. Last inning, down by a run, you’re up third. You’re hot. We get somebody on ahead of you and you poke one out, we win. You gotta stay.” Kulisek is so mad his face is on fire. Who cares? He’s always mad. That’s why I never hit against him. He’d probably kill me.
I pull off my catcher’s mask, dump it with the mitts and bats in the dust by the bench. “I’ve gotta go,” I tell everyone.
“’Whaddya have to catch up on your beauty sleep?” sneers Kulisek. What a jerk.
“I’m outta here,” I tell him.
As I leave, I hear Kulisek say, “Forget it. Somebody else can catch.”
I hear Freck say, “Not like she can.”