17-year-old rapper Chief Keef is already getting a head start on his baby mama drama -- in fact, he's so far ahead of the game ... he allegedly impregnated a girl who's STILL IN MIDDLE SCHOOL ... and now, she's suing for child support.

The girl filed the petition for child support in Chicago, claiming Keef -- real name Keith Cozart -- fathered a baby daughter with her back in 2011.

The girl's exact age is unclear, but according to the docs, she currently attends a Chicago middle school -- meaning she was .. at MOST ... in 8th grade when she conceived the child, but could have been in 6th grade.

Cozart -- who has since worked with Kanye West, Young Jeezy, and Rick Ross -- was 15 at the time... MORE

BUT DID YOU KNOW ...

In Illinois, it is considered a misdemeanor for two people under the age of 17 to have sex ... even if they're in a consensual relationship. When both parties are between 9 and 17 years old, the older person commits a crime called "criminal sexual abuse."