Full disclosure - I started out as a High School English teacher. I also had certification in Middle School English, MS / HS Math and MS/HS Physical Education, and I later gained ESL / Sheltered English / ESP qualifications. However, it was only when I became a principal that I realized the problem with English.
Math teaching is easy to define and thus to limit - you teach Math. While defining "math" presents challenges, a math teacher is just that. Similarly, a PE teacher teaches PE (although to be fair, for a while PE included Health but that seems to have been just a phase).
However, an English teacher teaches three or four quite different, although overlapping, subjects presented here in no particular order. The fourth of these is the problem.
Firstly, an English teacher teaches the study of English Literature, or literary analysis and response. The first requires a knowledge of different texts, styles, genres, epochs and so on. It requires pulling together a line here, a detail there, a hint somewhere else. It also requires a balance between text-centered (cf Supreme Court constructionism) and reader-response approaches. The second requires the ability to take a position, "this is what the text suggests", and to argue it in writing and or in speech.
Secondly, an English teacher teaches the study of English Language, or what English means in a social context such as advertising, persuasion, propaganda, non-literary texts such as instruction manuals and autobiographies. It requires semantics, sociolinguistics, philology, and considers analysis of and response to spoken and written texts. Again, students must be able to take a position, "this is what the text suggests", and to argue it in writing and or in speech.
Thirdly, an English teacher asks students to consider what it is to be human. By looking at what others have written and said, both literary and non-literary, students are confronted by views and values which may be quite different from their own. Consistently, students must be able to take a position, "this is what I believe", and to argue it in writing and or in speech.
Such a contrast in views and values is inevitably controversial, and the
cause of many disputes with English programs, teaching, and teachers.
Yet this is not the problem of English.
These three different but overlapping subjects should require extra time and resources. However, these are rarely, if ever, given. Interestingly, England does seem them as two different subjects thus offering two different school-leaving examinations at age 16 - GCSE English Literature and GCSE English [Language]. The lack of time to cover all three subjects, and to do so in terms of inputs (analysis) and outputs (response), is not the problem of English.
The problem of English is that English teachers must also teach the Use of English. By Use of English I mean both the mechanics of vocabulary, grammar, punctuation, spelling, pronunciation, and the methods or written and oral expression such as essays, speeches, groupwork and even poster design. The problem is that Chemistry teachers, Math teachers, PE teachers do not see teaching the Use of English as within their job description. So they accept poorly-written assignments, babbling and incoherent explanations, lengthy and inefficient instructions in activities, and frequently do so while bemoaning the competency (or its lack) of their English teacher colleagues.
Until all teachers see themselves as responsible for teaching the Use of English, we will continue to have the problem with English.