Well, considering one of Walter Dean Myers' mantras was "Reading is not optional," any reader can be certain that staying in school was something that Myers' thought was very important, too. This is especially true in regards to Fast Sam, Cool Clyde, and Stuff. Here are three specific quotations about staying in school that you asked for with explanations.
Well, if you make something of yourself, and all that kind of thing, you'll be doing something. That's what you got to do. Ain't really nothing else you can do except living like he was still alive and being good to your mamma. All that good doing stuff. What are you planning to take in college?
You see, Clyde has just lost his dad and, in the midst of being very sad about that, Clyde is trying to figure out how to become his dad's legacy. When Clyde says he wants to "make something" of himself and that he wants to be "doing something" and that he wants to be doing "good doing stuff," Clyde is specifically talking about staying in school and making that his job, especially going to a university. This would be living like Clyde's dad was still alive in that it would make Clyde's dad proud of him.
Here is another quotation in regards to what Clyde asks and what Fast Sam replies:
"Look, you think I can make a college program?"
"Yeah, I think so. And even if you can't, I can think you can scare the daylights out of it if you don't get it."
What is interesting here is that Fast Sam knows he is not as smart as Clyde and, as true friends, Fast Sam doesn't try to negate Clyde's ability. Instead, Fast Sam encourages Clyde to do what he is able to do. These are words of encouragement and, again, can specifically be translated through Myers' words as encouragement for readers to stay in school.
Finally, there is another quotation by Clyde that is pertinent here:
I will have to give [college] that big try. My mother would dig it, my father would have liked it too.
Again, as a main character in Fast Sam, Cool Clyde, and Stuff, Clyde here decides without a doubt to go to college in honor of his mom and dad. Myers, as the author, wants his readers to learn from this as well. Education is key and "reading is not optional."