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]
(..) And what a beautiful morning it is! Back to work, I'm afraid. It's adventure time. Egypt again? It's nothing but pyramids and sand. I know. Gets everywhere, in the cracks... Spanish galleon? Do you know what day it is, Hillary? Yes, of course... The th. And that is never a good day. (..)2001 Lara Croft: Tomb Raider