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]
(..) Stella... take those binoculars out and bring them here, will you? Trouble. I can smell it. I'll be glad when they crack that cast, and I can get outta here. Uh-huh. How far does a girl have to go before you'll notice her? If she's pretty enough, (..)1954 Rear Window