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?
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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.