Hi Tablo, we use the word 'grade' in American English to refer to the year that students are in - ex - The 6-year old boy is in 1st grade. A 'stage' usually refers to a level - ex - first stage, second stage, last stage of a project.

In the UK, 'stage' isn't used for this at all. 'Grade' is understood but we use 'year'. "My son is in year 10", "When I was in year 7 ... ".

The U.K. words ("level", "year") are different from the U.S. word ("grade") but one thing we still agree on is never to use "stage". It is used for theatre and, as JK noted, for psychology (e.g., "Billy is going through a stage where he is starting to notice girls").

"Stage" isn't a term from the world of education. In Developmental Psychology, "stages" are periods of development in childhood. Notable developmental psychologists who spoke of development in "stages" included Freud, Piaget and Erikson.