Rihanna -- Four Hours Late to Charity High School Gig

Rihanna made 2,500 students wait for over FOUR HOURS -- crammed in a tiny high school gymnasium -- before finally showing up for a mere 16 minutes, this according to local reports.

Rihanna was slated to appear at Barrington High School in Chicago after the school won a charity competition put on by the pop-star. The appearance was supposed to start at 1:00 PM, according to The Chicago Tribune, but RiRi didn't show up until well after 5:00 PM.

The singer tweeted from her car on the way to the event ... "This Chicago traffic is not working."

When Rihanna finally did take the stage, she told the kids, "Thank you guys for coming and waiting," but reportedly made no other explanation for her extreme tardiness.