When did the American educational system start teaching evolution?

I'm not sure exactly. It was sometime after the Scopes Trial (1925). It's always been controversial with some in the extremist religious community, but that's fluctuated over time. In the '60s, the public had a lot of confidence in science, since that was taking us into space, and it's what was letting us beat the Soviets to the Moon and so on. But since then the public has been getting away from confidence in science, sadly. In the 80s, creationism was rebranded as "Creation Science"--pushing creationism by using scientific-sounding words. The Supreme Court exposed and struck that down in the Aguillard case (1985 if I remember right). At that point the organization rebranded itself as Intelligent Design and began gathering for a push that it started to make around 2002-3.

I think this has been an issue for a very long time, but it's just been getting an increased amount of press coverage in the past decade or so. There really was no point in time that schools "started" teaching evolution as opposed to something else... They most DEFINITELY were not teaching creationism at any point in time, contrary to what the second answerer suggests, because that violates the establishment clause of the U.S. constitution.

Teaching evolution in some form or another in public schools probably goes back at least a century, because Darwin originally proposed much of the basis for evolution 150 years ago. Schools generally teach whatever is the consensus scientific view of the time, so evolution in schools has probably been around a while. In fact, I can guarantee it has been part of the normal biology curriculum for at LEAST 50 years, bare minimum. I think what we're seeing in the press are the results of a more organized movement to push the teaching of creationism in schools, rather than any significant change in the schools' curriculum.

Around 1930.

This was settled by the courts previously but then the religious came up with the Intelligent Design idea which they tried to use as a crowbar to get into science classes back around 2000....where it was shot down again.

The matter is not in any real debate as far as science goes....the only place it is a struggle is in towns where religious school boards are trying to put religion and non-science material into science classrooms

Not sure of the year, but keep in mind that until the 60s, the teachers led their classes in prayer before they started the day. My guess would be sometime during the 60s the theory of creationism sort of disappeared from the curriculum and evolution was the only theory left.