The 3 women tore down the BLM art hanging in front of The Cottonwood School on Sunday, while one of them -- appearing to be wearing a bulletproof vest -- expressed her disdain for the messaging on the signs.

According to the school ... the woman recording the incident is a community member, and despite her pleas for the 3 women to stop -- or at the very least not steal the kids' signs -- they ripped it all down and took off ... but not before the woman in sunglasses had a freak-out.

The good news -- the video's making the rounds, and members of the Portland community gathered Tuesday to support the students at the K-8 school with brand new signs ... while the students are working on making more of their own.

Cottonwood says that it "stands in solidarity with our Black families. Our goal is to nurture compassionate and confident students who possess the skills needed to confront racism in our community and beyond."

It continues ... "We are committed to fighting systemic racism, especially within our schools and neighborhoods, and we strive to model humility and non-violence for our children."

By the way ... the original artwork hung by the students went up in June. The school says the Oregon State Board of Education recently passed a resolution encouraging districts to support Black students, educators, and community members ... including renewing their commitment to anti-racism.