Howard Zinn has done more damage to this country than almost anyone else.
I think you may be exaggerating his importance. He is just one of many horrible people who collectively moved American culture toward the radical left (he himself was outed as a member of the Communist Party), but I think that other than the leftist teachers and a small coterie of students, nobody took him very seriously.
In one of my daughter’s high school classes his book was assigned as summer reading for a US history class but never discussed in class. I suspect that this is the case in many classrooms where the leftist unions and administrators think that they control more than they actually do.
This article suggests to me that the left raises massive amounts of money by requiring students to buy this book, which nobody would buy on his or her own, and then funnelling the money into leftist causes.