My 14 year old granddaughter says they could teach what she learned in half the time if teachers were efficient.
as a homeschooler, i have found this to be true... with my oldest son, his first eight years of school (K-7) were done in 2-4 hours a day, 3-4 days a week... (2 hours in the early years making its way to 4 hours as he got older)... he is now in eighth grade and is doing a full day of school (a good 7 hours) 4 days a week, about 8 weeks a quarter... we go really hard for 6 weeks, then take the learnings from those 6 weeks and polish them for another 2-4 weeks... and then start the next quarter....