First and foremost, you want to place your child in a school where being smart is cool. The minute your child enters any school culture where 'goofing off', cutting class, or talking back to grown ups is highly admired by most peers, you will be swimming upstream until you get your child out of that setting. Unfortunately, you will not know if this is true about your school until you are IN the school, yet, it's a very important factor to look for and one to not ignore for sure.
Second, if you have a son this next one will be of most importance. I believe gender-balanced schools insure less bullying and help grow more expressive, emotionally-balanced and self-accepting boys; and girls too of course. Girls are usually more flexible in the social-emotional arena than boys. So as a mother of a boy, I have to deliberately make choices to raise my son to express himself emotionally and to embrace himself fully and that can only truly happen in a gender-balanced setting. In schools where extra-curricular activities are gender-specific (such as where only boys play football for example) the danger lies in the absorption of very finite, rigid, and stratified gender roles for both sexes. These rigid distinctions lay the groundwork for higher incidences of bullying and unhealthy social interactions. As a parent, determine whether your school is gender-balanced would be easy to determine before walking through the school doors. Ask around, do boys and girls have soccer teams? are there any boys in knitting class? check their website, etc.
Third: The classroom teacher. The classroom teacher can make or break a child's year, period. There is no magic formula for knowing how this relationship will pan out in advance, but use your child as a barometer to determine if this relationship is working for him/her. Don't be afraid to advocate for your child if you notice things are not working out. You can be at the lowest rated school, but if the teacher has truly connected with your child in a positive way, your child will learn a lot more that year than years where the connection was lacking. It's been my experience that human connections are the most powerful agents of growth and change, therefore, a positive teacher-child relationship should be every parent's top priority when choosing a school.
Now, here is my disclaimer. I am absolutely not one of those parents who worry about college while my kids are in elementary school. I am not trying to shape their entire future by 'playing the field' or strategically stuffing their days with clubs to make them more college-ready. Instead, my main goal is to influence their future happiness and personal success by making sure they have a magic-filled childhood with unconditional love, constant support, a nurturing environment and most importantly, our time. When looking at any school, my child's social-emotional needs are first and foremost always. Having worked on myself for years to overcome my rocky beginnings, I feel very strongly about the importance of an emotioanally balanced childhood. One could be the richest person in the world, or even the most powerful and still be at war in their heads daily. This is so because we ultimately only are where our minds are. Having our mind be in peace and open to all that life has to offer is THE ultimate marker of success in my book.
Parenting should mirror agriculture in that you can't control the outcome of this child you have before you. No matter how hard you try, you can only set the groundwork for good growth and allow the rest to come from them.