Disadvantages: Parents sometimes assume that a private school is religious in nature. If this is what you desire, you may be sadly disappointed! A private secular school has a different approach to education that the public schools, but they are not Christian in their foundation or philosophy. Be sure to read the school's philosophy and statement of purpose carefully.
Often, but not always, your child must 'qualify' before he/she can be admitted. This is usually an academic qualification. Some private schools will only take children who are considered above average or academically gifted.
Prestigious private schools tend to draw families from a higher socio-economic income level than average, which again limits the parameters of who is able to attend.