Porn legend Sasha Grey -- winner of the 2010 AVN award for best anal sex scene -- was invited to read books to 1st graders at an L.A.-area elementary school last week ... but the school tried to cover it all up ... after parents pitched a fit.

TMZ has obtained photos of Sasha reading children's books at Emerson Elementary School in Compton on November 2nd ... participating in the Read Across America program. Grey, who hasn't done porn in 2 years, may have been invited because she's mainstream now, with credits which include "Entourage."

Sasha tweeted about the experience -- calling the students the "sweetest" ever. However, not everyone saw it that way ... some angry parents complained to the PTA -- who then contacted the school's principal ... but there's one problem.

A rep for the school district is flatly denying Sasha was ever inside one of its classrooms -- telling TMZ, "We have several celebrities who read to our students each year. The actress you have indicated [Sasha] was not present."

Clearly, the photos we obtained show otherwise.