In the United States, the word "doctor" is used primarily in two ways. Most commonly, "doctor" is short for "medical doctor" is a physician who treats illnesses. A medical doctor commonly has an MD degree from a medical school. Less commonly, a "doctor" is someone who has earned a specific degree, the "Ph.D.," from a university. It implies not only an advanced level of knowledge in a field but the ability to do new work in the field. A Ph.D. might be in mathematics, or economics, or history. In this case, we use "doctor" as an honorific, when addressing them or introducing them. For example, "The director of the Hayden Planetarium is Dr. Neil deGrasse Tyson." However, we would not say "doctors" when referring to a group of Ph.D.'s, we would call them "Ph.D.'s" Sadly, we do not have an honorific for high-school teachers. We simply use the everyday honorifics--Mr., Mrs., Miss., or Ms.,--or no honorific at all, depending on the teacher's preference. We would only address them as "Dr." if they hold a Ph.D. degree in education, which is rare.

Interesting note, in the following sentence when it says "as late as 1841," that implies that may have been the last time it was used that way, and even then it may have been non-standard. However, in the non-medical sense, it still does essentially retain its meaning of wise and respected scholar. It's just that it also requires a degree, which is supposed to represent those qualities. The one thing it no longer means is teacher.