In the context of language-arts education, a certain prescriptivism is unavoidable. There is a written standard, like it or not, and there are social consequences to violating the standard in certain contexts.
The traditional understanding of grammar—the one we associate with the prescriptivist position—began in ancient Greece and Rome. For hundreds of years, grammar was synonymous with the study of Greek and Latin. These languages were regarded as perfect—or nearly so—and their grammatical structures were taken to be universal forms by which all "vulgar" languages should be judged. It was not until the seventeenth century that writers began to turn their attention systematically to the grammar of English itself, and when they did so, they applied the structures that they had learned studying classical languages to English.
Notice that there is a place for both description and prescription in language study. For example, when adults learn a foreign language, they typically want someone to tell them how to speak, in other words to prescribe a particular set of rules to follow, and expect a teacher or book to set forth those rules. But how do teachers know what rules to prescribe? At some point in time, someone had to describe the language and infer those rules. Prescription, in other words, can only occur after the language has been described, and good prescription depends on adequate description. We obviously don't want to be teaching people the wrong things about language.