Please if you can, go to the forward in Stephen Jay Gould's book about that. He firmly and clearly declared in the Prospectus of his book that he learned Haeckel's theory in the New York City school system FIFTY YEARS AFTER scientists had abandoned the theory. So after scientists gave the theory up for error, the school system(s) were teaching it. Clearly the school system was teaching it to unsuspecting students. Later, I suppose, he realized there were serious flaws in it, and he felt the need, despite walls of opposition, to investigate this further. In other words, it most certainly was taught as fact in New York when Gould went to school there.
I am reading the book in parts, and perhaps we'll see what Gould's take is on this issue in the final analysis. But if you note, I am not discussing right now if it's true or not. I am saying that it was taught definitely as truth to New York State schoolchildren. Many of whom believed it wholeheartedly without reservation or thought for the rest of their lives. Unlike Gould, who had the temerity and nerve and decision to examine it more closely later on. Good for him.