I definitely think that it is important for students to learn formal grammar. I do not see why anyone would argue this. Formal grammar is important for a lot of people in terms of speaking in the real world and professional business place. I also think that students should writer papers like them as well.
Yes they need to learn formal grammar because once they are out of school if you can't spell and pronounce words correctly in a job interview that employer will not want to hire you. The slang is good for sometimes but formal grammar is way more important than you think.
With the development of the Internet, cell phones, and the like, the world has become an increasingly digital -- and text-based -- place. Now, more than ever, it is important for students to learn formal grammatical rules so as to not come off as unintelligent, whether it be in personal or professional relationships.
All forms of language can be considered valid if a fairly broad set of criteria are met, but formal and proper grammar are important in order for people to demonstrate their ability. While grammar might not seem important to some, a common standard is needed in order for students to demonstrate they can perform at a certain level.
I just came across an interesting poll from the BBC. More teachers like students that work out the grammar rules themselves, than writing like a book. They rather their students focus on the content of what their writing, instead of getting bogged down with where every little comma, in every little spot goes. I have to say I agree.