Still, Schultz fought for Franklin's inclusion in his strips. From Wikipedia:

A Los Angeles schoolteacher named Harriet Glickman wrote to Schulz on April 15, 1968 (11 days after the assassination of Martin Luther King Jr.), urging him to introduce a black character into Peanuts.[4] On April 26, Schulz wrote back, saying that he had thought about this, but was afraid of “patronizing our Negro friends.”[5] This began a correspondence between Schulz and Glickman that led to Schulz’s creation of Franklin.[6][7] In an interview in 1997, Schulz discussed receiving a letter from a Southern editor “who said something about, ‘I don’t mind you having a black character, but please don’t show them in school together.’ Because I had shown Franklin sitting in front of Peppermint Patty... I didn’t even answer him.”[8]

In a 1988 interview, he recounted other negative reactions towards the integration of Franklin with the white characters. “I finally put Franklin in, and there was one strip where Charlie Brown and Franklin had been playing on the beach, and Franklin said, ‘Well, it’s been nice being with you, come on over to my house some time.’ Again, they didn’t like that.” Schulz argued with Larry Rutman, president of King Features Syndicate at the time (distributor of Peanuts comic strips to newspapers). Schulz said, “I remember telling Larry at the time about Franklin—he wanted me to change it, and we talked about it for a long while on the phone, and I finally sighed and said, “Well, Larry, let’s put it this way: Either you print it just the way I draw it or I quit. How’s that?”