Why was Darry upset with Ponyboy? What Soda`s reaction?

after supper, Darry and Ponyboy get into a spat about Ponyboy's grades, which have dropped significantly. But Darry says that "schoolwork's not the point. You're living in a vacuum, Pony, and you're going to have to cut it out." He says that they're all upset about losing Johnny and Dallas, but that Ponyboy has to come back to reality.

When Ponyboy looks at how Soda is reacting to the fight, he sees that his face is white. Then Soda runs out the door, dropping an envelope. It is the letter he wrote to Sandy, returned unopened.