Charlie Sheen -- Private School Goons Threatened My Kids ... With a Knife

Charlie Sheen is now explaining why he was so pissed off today ... telling TMZ Live, a couple of lackeys supporting his daughter Sam's former private school left a threatening note and a knife jammed into a tree outside his daughters' home.

As we reported, Sheen posted another angry rant on his Twitter today, claiming some spineless "doosh bagz" connected with L.A.'s expensive Viewpoint School had terrorized his ex-wife Denise Richards and their 2 girls.

Charlie called in to TMZ Live ... saying his daughters "had the pleasure of discovering a steak knife used to nail a note to the tree." Charlie says the note contained a threat directed at his family.

Charlie says two people were captured on surveillance tape committing the deed -- and he's contacted police.

Charlie also modified his initial call to arms -- in which he called on his fans to smear dog crap on the Viewpoint School -- saying it was "metaphoric."