Why would the narrator say "most"? Are there people who graduate the school either early or late?
Answer from a Native speaker
Great question! He uses "most" because there are some individuals who do not graduate high school. Some students drop out of school for various reasons. Or some students decide to further their education by going to university or a trade school. This could mean that some may not finish school until their late 20s or early 30s.
1/2 STEP LEARN