Both descriptive and prescriptive grammar are great to be applied in teaching grammar. They simply refer to how grammar learning should - or is better to - be delivered.
Descriptive grammar simply focuses on how particular sentences or expressions are used. It deals with functional level of language processing - the purpose for which the speakers are using the language.
If someone is in a job interview or is writing academic journal, the use of slang is inappropriate. The interviewee or the author should use formal expressions. Descriptive grammar plays its role in telling the speakers to use appropriate expressions in particular situations. Another example, if someone meets some friends outside the classroom - in canteen, it would be strange to greet friends by uttering formal expressions such as "ladies and gentleman". He can simply greet his friends by using 'guys', 'friends', 'mates', or 'buddies'.
Prescriptive grammar simply focuses on correction. It emphasizes the teaching of formal level at which grammatical errors made by the students are corrected. Prescriptive grammar deals with grammatically 'correct' or 'wrong' - such as in the following examples.
- She have to go to campus
- She smart
When prescriptive grammar dominates, the teacher will - either directly or indirectly - correct the grammatical errors on both sentences. The teacher would correct them with explanations.
- she has to go to campus. The third singular pronoun should be paired with has instead of have.
- she is smart. Missing to be. The correct be verb of she is 'is' if the context is in present and 'was' if the context is in past.