Neville nodded nervously, but Moody made no further inquiries. Turning back to the class at large, he reached into the jar for the next spider and placed it upon the desktop, where it remained motionless, apparently too scared to move.
This is a scene from Harry Potter And the Goblet of Fire in which Moody showed three unforgivable curses to the students. After Moody had used a curse to torture the first spider, Neville Longbottom gave the second correct curse name:
the Cruciatus Curse and Moody is poised to use it on the second spider.
"At large" has the meaning:
at liberty; escaped or not yet captured.
"Turning back to the class at large" seems to be saying that Moody tortures spiders cruelly without fear of punishment?