Charlie Sheen And Denise Richards -- Our Kids Are Smarter than a 5th Grader ... So Now the Tutor's Suing

Charlie Sheen and Denise Richards are about to learn a hard lesson from some very expensive tutors .... in court.

The warring exes hired A-List Tutoring Services -- seriously, that's the name -- to help daughters 11-year-old Sam and 10-year-old Lola gain an edge. The 4-week bill totaled $11,156.25 ...seriously.

Guess who A-List claims didn't pay?

It looks like Denise hired the tutors, because she's listed as the primary defendant and she also has custody. But Charlie's a parent too, so they're both being sued.

No comment from Charlie or Denise.