I believe it is NOT “Buon lavoro” because an Italian person online expressed that this was to wish someone a good day at work.
So - what do teachers say to students to let them know they've done something well?
is a useful expression used to indicate that you are satisfied with a job/ task which have been done well.
Teachers say 'Ottimo!' or 'Eccellente!' or 'Ottimo lavoro!' or 'Lavoro eccellente!', but they also say 'Buon lavoro!' when a job is a 'Good job!'
Just to add to the other excellent answers: a simple translation as
is also very common. So, good becomes beautiful/nice in the translation.
On the other hand, "buon lavoro!" is used as an encouragement to someone who has to perform a task and you want to with her success.
A common Italian expression is "Ben Fatto," or "well done." Here, the emphasis is on the adjectival modifier, "ben."
Other expressions are "Bravo," "eccellente," or "ottimo," which are also adjectives.
"Bel lavoro" is used, but is less common, because it would put the emphasis on "job," rather than "well" or "good."
Bravo/Brava doesn't mean "Good job", it's more stating a personal opinion about a person who did a good job, so "brava/bravo, because of the way the job was completed".
The best way to translate "good job" in Italian is, imho, ben fatto: it sounds like "properly done", so stating something related to the way the task was completed.
The equivalent for "Good Job" is Bravo or Brava, depending on who you're talking to. And you could simply emphasise the same by making that Bravissimo or Bravissima.
Then there are many other words, Ottimo (Excellent), Ben fatto (You did well) and a few others. Easiest to use is Bravo.
I think that the closest Italian translation of "Good job!" is "hai fatto un buon/ottimo lavoro"
I think that the best is "Ottimo lavoro!"... I always use this (I'm Italian).