First you need to accept the fact that the  public schools are a substitute for daycare for many parents.  Daycare in Oregon is running about $10,000 per year according to an OSU article.  "Families making under $28,000 a year are spending 29 percent of their income just on child care".  " The average minimum wage worker is spending almost 60 percent of their income on child care."   Adding four more days a month to daycare for those who need it would be very difficult.  

Zachary may be thinking just about high school age students, he doesn't say.   The number of high school students who would use an unsupervised day off productively is unknowable.