A preschool education hurts children at school (and in life) when the scope of instruction is too narrow and the goals too limiting. A good preschool is all about expanding the child's universe through play, hands-on learning, exploration, and socializing. A good preschool teacher wants her students to get energized about learning, knowing it comes from their own curiosity and is not spoon-fed to them by an adult.
Unfortunately, play-based preschools are dwindling. Because of the academic rigor in the primary grades, parents want their kids prepared for kindergarten. This preparation includes writing in workbooks, sitting still for long teacher-directed lessons, learning letter names and sounds, and learning how to count and recognize numerals. There is no evidence that teaching these skills early has any long-term benefit. There is evidence, however, that it causes stress and turns kids off to school. We are seeing higher rates of depression and anxiety among children and the lack of play in early years is one likely culprit.
Parents and teachers need to look at the big picture of education so kids will become life-long learners, knowing how to think critically, work cooperatively, and solve problems in creative ways. These are all more important than knowing that "t" makes the "tuh" sound.