English prof up on a soapbox here.
Comments like “you have to learn the rules before you break them” are kind of missing the mark. For the most part, those of us who teach English/composition/literary criticism/etc aren’t particularly concerned with “rules” (with the exception maybe of English as a second or foreign language, where conventions/rules are used to scaffold learning rather than as universal truths). English teachers are familiar with linguistics and the fluid, messy nature of language and all the fun things we can do with it. The “grammar Nazis” you encounter online are VERY unlikely to be English teachers (though they likely were our most insufferable students), who are often the first to point out that language is constantly changing and that what’s considered “correct” depends heavily on … everything, really.
But we’re talking here, ostensibly, about a “class.” And if we’re talking about a class, we’re talking about a syllabus. Specific outcomes on which a course is responsible for improving and checking proficiency. Your English class isn’t necessarily about “learning English,” especially if English is your first language. It’s about getting you to become proficient at doing a specific selection of specific things with your language. Very often these things aren’t even about rules or grammar, but that doesn’t mean the expectations aren’t strict- and what most people read as strictness is usually just specificity.
It’s damn near impossible to know whether a curriculum, teacher, method, or activity is successful without an outcome to measure. Lack of specificity makes measurement difficult and potentially unfair. If you’re told to “write a persuasive essay,” what exactly does that mean? What separates an A from a C? How does the teacher justify that distinction, and how does the student know what they need to improve? Without very specific target structures or a “can do” inventory, assessment becomes messier and we leave students guessing about what an given teacher might want from them, which isn’t especially fair (or useful, from a pedagogical POV).
A teacher can understand perfectly well that language is flexible, that different contexts call for different kinds of language, and that conventions change over time, while still expecting students to demonstrate a particular skill according to given criteria. There isn’t one correct way to use English, but a class DOES needs to establish what it’s trying to do, what students are expected to be able to do, and how anyone can tell whether they’ve been successful. That’s not necessarily prescriptivism, it’s just how a curriculum works.
I guarantee your writing class has a syllabus. The problem here is a disconnect between what you think the syllabus says and what it actually says (this may not be your fault- a lot of teachers are notoriously shit at communicating the purpose of the syllabus).