CHIEF KEEF SUED BY HIS BABY MAMA FOR CHILD SUPPORT!

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.

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."

Punishment for the crime includes a max sentence of one year in jail ... and a requirement to register as a sex offender for ten years -- but in reality, these cases are almost never prosecuted.

The most likely scenario in which Keef would face a judge is if the girl's parents press charges -- but if that hasn't already happened, it probably won't.

The girl is suing Keef for an undisclosed amount of child support, including health insurance and other medical expenses. Calls to both Keef's people and the girl's family were not returned.

Keef's no stranger to legal problems -- he was just sentenced to 60 days in juvey for violating his probation on a gun charge.

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