Nilaja Sun’s one-woman play “No Child,” currently being staged at the American Repertory Theatre in Cambridge, is a phenomenal mix of storytelling, journalism, activism, and civic engagement. The 70-minute performance isn’t bogged down by intricate sets, costume changes, or dazzling lighting design. Instead, Sun uses three chairs, a broom, and her ability to embody and humorously animate a dozen characters to start a meaningful conversation about education reform. The fact that it is as tech-free as a theater performance can get without leaving the audience literally sitting in the dark should serve as a lesson for everyone aiming to use new technologies to facilitate social justice.