I'm not sure it's the simple dichotomy that JTT's (prescriptively descriptivistic?) comments suggest. Yesterday's prescriptions are today's descriptions. The first prescriptive grammarians we encounter are our parents. From their speech patterns, and their corrections to our early efforts, we build up rules.
Then we listen to the speech patterns of other people, especially those of our peers, and adjust our own speech accordingly. We become fully-fledged descriptivists. Meanwhile we are introduced to books: the written word begins to influence our speech, descriptivistically, while our teachers' commentaries add further prescriptions.
Then we go to work, and learn new, specialist dialects: mostly descriptivistically; but sometimes, often unwillingly, by prescription. We learn the true meaning of 'bad grammar': you don't get the job.
Then we have children, and become prescriptive grammarians ourselves:
not 'eleventy-one'! '121'!
For ESLs, the process is much the same: perhaps with even more emphasis on the 'prescriptive' side. It doesn't matter if children say 'eleventy-one' (1000 hits on Google). But non-native adults have to be a little bit more careful.
MrP