Before the 1940's, the calculating devices were mechanical. It wasn't until electronic parts were introduced into calculating devices that they became known as computers.
What is considered to be the first computer was made in 1944 by Harvard's Howard Aiken. His Mark1 computer was very much like the design of Charles Babbage's, having mainly mechanical parts, but with some electronic parts. The machine could automatically solve equations and prepare mathematical tables by performing arithmetic operations.
One of the first all electronic computers was the ENIAC (Electronic Number Integrator and Calculator), a general purpose digital comptuer built in 1944. ENIAC contained over 17,000 vacuum tubes and was 300 times faster than the Mark1. John Mauchley and Presper Eckert were partners in creating the ENIAC.
According to a 1973 court case, the title and prestige of being named the first all electronic digital comptuer was awarded to the ABC, the Atanasoff Berry Computer. John Vincent Atanasoff and Clifford Berry created and bult the comptuer in the bottom of the Physics Building at Iowa State University in Ames, Iowa. The idea was created by Atanasoff in 1936 and both men built it. World War II interrupted their progress and the filing of a patent with the U.S. Patent Office.
In the 1960's and 1970's, computers were made smaller by replacing the large vacuum tubes with transistors and then integrated circuits. In 1977, the Apple Company, unveiled the Apple II which became the first popular home computer. This companhy marketed their computer to education. It was in the late 70's that schools began to put computers to use in classrooms.
Read an article on Dr. Atanasoff as the "Forgotten Father of the Computer".