It's impolite to pick your nose in front of people.
Corey is at lunch with his friends when he feels an uncomfortable blockage in his nose.
Corey wants to clear his nose.
Corey excuses himself and goes to the bathroom and blows his nose.
Corey picks his nose to get the blockage out while his friends are talking.
Corey clears his nose and doesn't give his friends any ammo against him.
Corey's friends notice and are grossed out and start teasing Corey relentlessly.