Parents are concerned that children who have trouble in school will 'fall through the cracks' in the school system. [=will not be given the help they need; will graduate without anyone noticing that they are unable to do the work]
The program is meant to help workers who may have 'slipped through the cracks' [=who may have not been included in other programs] because of their age.
You ought to 'get cracking' [='get going; get moving'] on that assignment. [=you should start working on that assignment right away]
(..) The bullet they just dug out of the wall's from a handgun. A kill shot over that distance, from that kind of a weapon, that's a crack shot. But not just a marksman, a fighter. His hands couldn't have shaken at all, so clearly he's acclimatised to violence. He didn't fire until I was in immediate (..)2010 TvShow: Sherlock Title: A Study in Pink Season: 1 Episode 1