From the Diane Ravitch blog:
Back in the olden times, advocates claimed that charters would provide healthy competition and beneficial innovation. This would benefit the public schools and urban districts as well.
But it turned out to be a false promise.
By skimming off the best students in poor neighborhoods, charter schools leave the public schools in worse condition, overburdened with the neediest students that the charters don’t want and lacking the resources to educate them appropriately. Where the public school system is struggling to improve, charter schools may actually hasten their collapse.