The question of separating English from Dutch has puzzled me a little - I also would like to know the underlying connotations of this one. Separating Wheat from chaff is, obviously, sorting out what is useful from what is useless: sorting sheep from goats is spearating mixed groups into like groups: separating men from boys is identifying those who are able from those who are not, in a particular field of endeavour.
But what is the criterion of separation in the English/Dutch analogy?

As to skipping school - that and all of the other ezxpressions, above, except goofing off are used in UK, with the addition of regional use of "to Mooch," e.g., "I'm mooching this afternoon." - in Wales it is "mych" (I think - mav?: but I think it is probably pronounced much the same) and in Ireland "mitch".