You must create a select element object from the drop down list.
using OpenQA.Selenium.Support.UI;
// select the drop down list
var education = driver.FindElement(By.Name("education"));
//create select element object
var selectElement = new SelectElement(education);
//select by value
selectElement.SelectByValue("Jr.High");
// select by text
selectElement.SelectByText("HighSchool");
More info here