"Exams are currently underway" meaning Exams are currently being taken and/or offered The building project is underway My dinner party is underway. Her training for the marathon is underway. Working is not a synonym. Synonyms would be "in progress" (My dinner party is in progress) or "has commenced" (Her training for the marathon has commenced)

"Under way" is similar to saying "in progress" - I wouldn't use "working" as a synonym. Like: Students are busy because exams are under way. Building construction is under way.