Quality of EducationMany people consider moving their children to private schools due to the perception that the overall quality of education, and their children's chances of getting into good universities, will be higher than that provided by public schools. For the most part, private schools do provide a higher quality of education (driven by other issues discussed below) than public schools do, but there are exceptions and it this is not always the case. Some public schools outperform private schools so it's important to look at the actual schools being considered to make sure you don't assume incorrectly.
FundingPrivate schools are funded through tuition paid by parents, endowments from graduates and parents of students, and to a far lesser extent government funds. The benefit of this is that often private schools have more funds available to put into teaching materials and facilities than public schools have. Schools with successful development teams and large endowment funds can rely on a steady budget to work with.
Public schools are government funded and a significant part of their budget is often tied to local taxes. As a result, schools in relatively poor communities will have lower budgets than those in affluent communities, and as a result many schools don't have the funds they need for facilities or teaching materials. With funds varying from year to year it is very difficult for public schools to plan for future years.
AccountabilityPrivate school teachers and administrators are incredibly accountable for the performance of themselves and their students. Poor results mean they could lose their jobs so every year they are pushed to ensure they deliver the best quality of education to their students. Private schools also tend to have less bureaucracy as extra staff mean extra costs and administrators are responsible for managing their costs. This often means they are more responsive to stakeholders (students and parents) and can adapt to changes in education practices and technology. Public school teachers and administrators are often pointed at as having a shocking lack of accountability where the number of years' service in the local union are rewarded more than performance. This lack of incentive for teachers means that students' education can often suffer.
The public school system also has a great deal of administrative bloat when compared to private schools. When you consider the funds and individuals committed to school boards, unions, school support staff, the total administrative body is enormous. The benefit these groups actually bring to education is limited, so the costs are effectively just thrown away money (from already limited financial resources).
Class SizeAs a result of additional funds available, private schools can often hire more teachers with the end result being lower class sizes. Extensive studies have shown that class size and the amount of one-on-one time between teachers and students has a significant impact on how students perform, making this a major consideration. Many private schools aim for a 15 to 1 ratio between students and teachers.
Public schools that are financially limited cannot hire as many teachers as they would like and this results in far larger class sizes, and the negative implications on education quality that comes with that. Many public schools have student to teacher ratios in the 30 to 1 or 40 to 1 range.