KIPP, a major charter school company writes :
KIPP teachers are expected to help all students succeed, and they typically work a nine-hour work day during the week, half days on selected Saturdays, and three weeks in the summer.(1)
These increased hours are not accompanied by increased pay, which generally matches the prevailing salary of the area. (i.e is 'competitive', as one will read in KIPP job adverts)
So why would one work many more hours for less pay ? While teachers generally are idealists, one can be an idealist in any academic setting. The bottom line is this model can only function in a highly depressed economy, where there is a large pool of college graduates with high debt and few job prospects.