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]
(..) We need to find out what he's up to. Trades... of some kind. Coded. And my fingerprint, courtesy of Selina Kyle. Get this to Fox. He can crack the code and tell us... what trades they were executing. I'll get this to Mr. Fox, but no more. I've sewn you up, I've set your bones, but I won't bury you. (..)2012 The Dark Knight Rises