Correctly written English could be called standard English. "Standard English" might be described as the English language as it is spoken and written by educated native users. Your English teacher is undoubtedly trying to teach you Standard English!
Standard English is the form of English which is most likely to be understood everywhere in the UK. As a commonly agreed dialect, it has many of the features of local dialects ironed out. This is not to say that words or sentence structures acceptable in local dialects but not in Standard English are always incorrect. Sometimes local dialects preserve much older versions of English, including ways of pronouncing words or conjugating verbs. For example, in certain parts of the UK, you might hear someone say something like, "That's me mam" for "That's my mum". Your first thought might be that the speaker is being ungrammatical by using "me" where Standard English would use "my". But, historically speaking, the word "my" was many centuries ago pronounced "me" and it is this pronunciation which has been preserved.
Nevertheless, it is important to master Standard English in order to be able to communicate well with the widest number of people, rather than only those who share the same dialect.
Check your knowledge of Standard English forms by taking this quiz.