She was supposed to introduce the band 5 Seconds of Summer, but Kendall Jenner, instead, exposed a HUGE secret to the Billboard Music Awards audience ... and to the entire nation ... and to everyone watching around the world: "I'm the worst reader," the reality show star said!

Hmm ... could that have anything to do with mom Kris Jenner pulling her kids out of high school and "tutoring" them?

Or maybe it has nothing to do with Kendall's literacy skills, maybe her slip up was just a function of having former flame Harry Styles on the brain. The model appeared to be saying, "One ..." instead of "5." So wait, maybe she just has a problem counting?