Back in high school...
A popular milestone in a person's life, during a time when they were a teenager. This phrase is used to show a contrast between back when you were a teenager, and now, presumably as an adult. It's also commonly used by an adult or older person to show how things are different from a person currently in high school.
Back in high school...I had to walk 10 miles to school, and have at least 5 hours of homework every night. It wasn't like today where kids have their own cars and have tutors.