someone who quits school before graduation
"After struggling to keep up with his classes, Marcus decided he couldn't continue and dropped out of high school last summer."
someone who withdraws from a social group or environment
"After years of attending every Sunday service, he decided to become a dropout when his new job required him to travel abroad on weekends."
Someone who has left an educational institution without completing the course
"After struggling to keep up with the advanced math classes, he decided to quit high school and became a dropout."