Being prescriptivist means not only describing the system of language to the students but also insisting that the students conform to using only what the teacher uses or what is presented in a course book. Students would then have errors corrected which are not necessarily ungrammatical but may be considered “non-standard” by certain speakers even though a large number of native speakers may use the very same forms themselves.
The world of TEFL has changed a lot over the years. In days when English was taught as a fixed body of Latin-based rules (e.g. “never split the infinitive!”), there existed a prescriptivist attitude. The position of form-focussed teaching has changed as has, with equal importance, the position of English as a world language. It is not correct or even practical, with a constantly changing, living language, to draw a line in the sand and state “here and no further.”