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]
(..) "if you want the rainbow, you have to deal with the rain." Even in his last few days... Um... He was always able to crack a smile. I knew this was the right thing to do. Funerals, I'd decided, are not for the dead. Hey, want some company? (..)2014 The Fault in Our Stars