Can't my child just make up for lost sleep on the weekends?

The first evidence of sleep deprivation is sleeping in on the weekends.
— Rafael Pelayo MD, Stanford University School of Medicine

Changing sleep patterns on weekends leads to a phenomenon that sleep scientists call social jetlag.

Let’s take the example of a teenager who needs to get up for school at 6 AM with difficulty but arises, feeling refreshed, a month into her school vacation at 9 AM. This implies three hours of social jet lag on every school day, equivalent to the jet lag of flying from San Francisco to New York five days a week. However, unlike traveling, there is no real habituation. Being jet-lagged every day sounds pretty awful, and it is the state the majority of our teens spend every day in.
— Craig Canapari MD, Director of Yale Pediatric Sleep Center