As the previous educators have commented, Clarisse suddenly disappears. While this news is delivered to Montag by Mildred in Part One, it is in Part Two that he really feels the force of this event. At the beginning of Part Two, for example, Montag says to Mildred:
"She was the first person I can remember who looked straight at me as if I counted."
Clarisse is, therefore, instrumental in guiding Montag towards a new way of thinking in which he questions society's norms and values.
In Part Three of the novel, when Montag is fleeing the Mechanical Hound, he is almost hit by a group of joyriding teenagers. This leads him to conclude that Clarisse may have met a similar fate. This is as close as the reader ever comes to learning what really happened to Clarisse since Bradbury never reveals her fate.
The simple answer is that she disappears. What happens to her exactly is more complicated. The book never fully explains it. Mildred says that she heard that the "whole family moved out somewhere", and that Clarisse was "run over by a car". Beatty hints that girls like her are "better off dead." So, she could have been taken away by the government because she was trouble, she could have ran away, or she could have been killed. The vague rumors that float around about her do help to keep it something that can't be pinned down and blamed though, which is handy if the government did take or kill her.
In the firt part of "Fahrenheit 451," Mildred tells Montag that Clarisse is dead. He wants to know if she is sure. She tells him she is not sure, but she thinks the girl got run over by a car. She tells Montag that the family moved out about 4 days ago; that Clarisse was killed four days ago. The second part of the novel opens with Montag reading to Mildred. He is reading from a book and he mentions Clarisse saying, "We cannot tell the precise moment when friendship is formed." Mildred again tells him that she is dead.