Yes, I did read the post. I have a G10 student who is similar. Has like a 4.3GPA. Missed one question on her AP Psych final, and just about died. My support for the student is to remind them that missing a couple here and there isn’t the end of the world, and that it was most likely a small mistake rather than a major shortcoming in their abilities. Now, a 15yr old with intrinsic motivation to get into an Ivy League is a little different than a 10yr old in grade 5, however, the concept is the same. Support what the student wants.
The belittling of others can be addressed by a school counselor or a child psychologist. In the classroom, one would expect the teacher to have adequate classroom management skills to deal. Give the student the chance to teach/coach a peer. Redirect the pride/arrogance into something healthy.