Language Variation & change
The English language is always changing and evolving. For all of its structure and systematic rules, it is also continually in flux, with the addition of new words and syntactical structures. Even among native English speakers, there is an enormous amount of variation. That variation is the result of regional dialects and social factors like age, gender, class, race and ethnicity.
It's important to note that linguistically, all varieties of a language are equal. In other words, no one form of a language is superior to another. Educators need to be aware that students may bring varying forms of English to the classroom. It is the teacher's job to honor the many varieties of spoken English and teach an accepted norm of spoken and written English that will help students succeed in the academic realm and the economic world, without discrediting the value of their heritage language and its linguistic value outside the classroom.
An interesting article about language variation and change is available from the Linguistic Society of America and is linked here.