Błędnie: You've been teaching adults for years, but you understand that young learners are a pretty kettle of fish. 

Poprawnie: You've been teaching adults for years, but you understand that young learners are a different kettle of fish. 

Idiom "a different kettle of fish" oznacza 'inną historię', 'inną sprawę'.

Z kolei idiom "a pretty (fine) kettle of fish" można przetłumaczyć: 'a to historia!', 'a to heca!'.