I read this short story in a compilation of short horror stories for kids back in 2005-06. One thing I do remember is the title of this short story was a combination of numbers and letters, like “SX458”, meaning that was the serial number for the robot-boy.
In the story, this kid befriends the school bus driver if I remember correctly, and later on we find out the kid is a robot and he is dying somehow? The lady bus driver then helps him recover. I don’t remember what happens next exactly, those are the only details I have.