The idiom toe the line means to follow the rules or act in the way people expect you to. If you show up to class on time and refrain from riding your skateboard down the halls, you toe the line at school.
When you toe the line, you acknowledge what has to be done to fit in with a particular group. If you're going to make it in the U.S. Army, you'll have to toe the line during basic training. The origin of this phrase is a mystery, but various theories include a school-related source: during roll-call or attendance, students commonly lined up with their toes touching a line on the floor.