Have the students listen to the song informally as they are coming into the classroom. This will give you a good idea of how well they know the song. In my experience, most students will have been familiar with the chorus and many will be able to identify with it as well at least at some level.
You open the lesson by asking them about the chorus of the song and how they feel about it. Who are the cool kids in their school and why? Elicit some ideas about what makes you cool from their point of view and what makes you uncool and write them on the board.
Help them produce some second conditional sentences with the ideas from the board, Such as:
If I were a cool kid, I would have many friends.
(If you need to, discuss the reasons behind If I was/If I were dilemma here or let students do some research on the subject online. This page is easy to find and offers a good explanation: http://www.cliffsnotes.com/cliffsnotes/writing/which-is-correct-if-i-was-or-if-i-were-and-why and fortunately is just one of many.)
Put the students into pairs or small groups. Play the song with lyrics and ask them to produce as many sentences as possible based on the lyrics. The will need to watch out for all the descriptions of cool kids and use them in sentences.
(If you have a competitive group, you can make it a competition.)
Students compare the produced sentences and check each other’s sentences.
Go back to the song and compare the cool kids from the song with the cool kids in your school. Discuss the differences and reasons behind them.
If you still have time, go back to the original video and ask if the singer is one of the cool kids or not and why they feel so. If you listen to the song again, in the second part it becomes clear that the singer is not singing from her own perspective but from a boy’s perspective, so all answers are correct here as long as they can support them.