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]
(..) Your ghost will only keep his men at bay so long. They've worked so far. You have a better idea? We can fight back. Did I crack your noble head? These are all good lads here... ...with hearts of oak, but they're farmers. It'd be lambs to the slaughter. They say the Sheriff (..)1991 Robin Hood: Prince of Thieves