Your main occupation is going to and completing HS.
Make sure you indicate that you can be claimed as a dependent by someone else in the "My Info" section where you edit your name. Miss that and you will severely PO your parents...and create a filing mess for you and your family. Don't rush to file, and print out your forms and have your parent(s) review them before you file.
The IRS couldn't care less what you put for occupation - so long as you pay your taxes. Here's an example of a few "occupations" I've seen on filed and accepted tax returns over the years, and the IRS didn't so much as bat an eyelash.
Occupation Real Job
Mass Murderer Pest Exterminator
Human Trafficker Modeling Agent
Serial Killer Pest Exterminator
Drug Dealer Pharmacy Technician
Car Jacker Worked for an automobile repossession company
Hijacker Aircraft Reposessor
Money Launderer This guy worked at the U.S. Mint. His job was to wash newly minted coins.
Terrorist Eradicator Navy Seal (I'm guessing he was on seal team six - but they won't say)
Computer Hacker Guy worked for the CIA – nuff said.
Overall, the IRS could really care less how you get your money - so long as you pay your taxes on it.