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]
(..) The air between the shells is untroubled by sonic emission or light wave. they don't seem to ask a lot of questions. The world's most decorated experts can't crack that one. The most plausible theory is that they chose places on earth with the lowest incidence of lightning strikes. (..)2016 Arrival