The answer to this question can be found in Chapter Five of this story. Pi relates how his unfortunate choice of first name, Piscine, sounded so similar to the word "pissing," that in his primary school he was mercilessly made fun of constantly. Often the teachers would join in this mockery of Pi as well, without realising that they were pronouncing his name in a slightly different way, much to the amusement of Pi's classmates.
As a result, when Pi leaves his first school and goes to Petit Seminaire, the private English-medium secondary school in his area, he has the chance to free himself from this legacy of mockery and bullying. He resolves to change his name and his identity, and when he is asked for his name goes to the front and writes his name on the board, before adding that he is known to all as "Pi Patel." He then works very hard on the first day to make sure that all of his teachers learn his new name and associate it with him, as "training humans is just as important as training animals."