What is the American expression for school leaver?

There isn't one single perfect equivalent to "school leaver" in American English, as the term's nuance depends on the context (age, type of school, etc.). However, here are some options, depending on the situation:

* High school graduate: This is the most common and straightforward equivalent if referring to someone who has finished secondary school.

* Graduate: This is a more general term and could refer to someone graduating from high school, college, or university. The context usually makes it clear which level is meant.

* Recent graduate: This clarifies that the person graduated recently.

* High school senior (who is leaving): If referring to someone *about* to leave high school.

The best choice depends on the specific context.