I have always been hesitant to use the term kurang ajar to describe recalcitrants and the ill-mannered amongst today's youth and would prefer to use the term kurang asam instead. I believe the former is a bit too strong and would unfortunately also reflect poorly on teachers and parents.
I was a pilot in Port Klang some years ago during a time when ship masters were pretty generous to pilots with complimentary bottles of whiskey and cartons of cigarettes. On Indonesian vessels in particular, we would often be rewarded with oleh-oleh of cartons of then very rare Gudang Garam kretek or clove cigarettes, which we would then distribute among pilot boat crew and office staff.
(Now, some would want to call this bribery and corruption, but lets not get bloody holier-than-thou about this).
So on occasions when I was piloting Indonesian vessels, some cheeky and expectant pilot boat crew member would invariably quip on the VHF radio: "Captain Yusof, ini rezeki bagus, pak. Mesti ada 'Gudang Garam'!"
Whereupon to avoid an awkward situation with the Indonesians, I would then retort with fake indignation and equally fake Javanese accent: "Gudang Garam nggak ada, dong! Gudang Asam ada. Goblok lu ... kurang asam!"
Hence the preference for the term, which should be about a notch or two below in severity...