I'm going to report the instagram account of a girl at school
Why do you want to report the Instagram account of a girl at your school?
I don't like the popularity she gets at school for doing nothing
You are being petty and nasty to even consider reporting an Instagram account for no reason. You have a jealousy problem and if you want to be popular you should address the reasons why you are not instead of going after someone else.
What if the instagram account is a threat to national security.
Then you should obviously report the account immediately to the authorities. Sarcasm aside, you won't gain any popularity and they won't lose theirs just because of your report. Be happy for their popularity and focus on your own life.