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]
(..) I take your horses to find my daughter. You are free to try and stop me. Business is business. Prepare five horses for them. I want everybody up at the crack of dawn. Yes sir. Understood cap. Carrying glass would be hard. We need to get ahead as far as possible. We'll divide into two groups. (..)2015 The Revenant