Japan. Top: One Month After Hiroshima, 1945. Bottom: One Month After The Earthquake and Tsunami, 2011. Incredible.
Are the images genuine, and stemming from the events mentioned?
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The first image can be seen at the Hiroshima Peace Media Center, in an article titled "Hiroshima: 70 Years After the A-bombing: Students study in open-air classrooms in “A-bomb desert”".
If you click on the first image, you get the following caption:
An open-air classroom at Noboricho National School. A female teacher stands in front of the students. The boy in the center of the third row, gazing at the camera, is Kengo Kurata. Far left is Fukuya department store, and far right is Hiroshima Nagarekawa Methodist Church. This photo was taken by Stephen Kelen, a war correspondent, between April and May 1946. (Mr. Kelen died in 2003 at the age of 91.) (Courtesy of the Hiroshima Municipal Archives)
This was not an isolated incident. From the Hiroshima Peace Memorial Museum Open-air Classrooms
In the shade of trees and out in the blazing sun, teachers and students could be seen diligently studying in classrooms made by placing the few surviving desks and chairs on straw mats on the ground. Open-air classrooms were common at many schools after the A-bombing, but they were not the pleasant break from regular classes we know today. The scene surrounding the students was utter devastation---the charred A-bomb desert punctuated here and there by the shells of ferro-concrete buildings.
The second photo can be seen in the Reuters article Japan makes no-go nuclear zone, PM faces more criticism, and was taken a month and a bit after the earthquake and tsunami in 2011
Nishitaga Elementary School children attend lessons in a temporary classroom at a school gymnasium divided by corrugated cardboard in Sendai, Miyagi Prefecture, April 19, 2011.
To summarise, both images are genuine, and depict the aftermath of the two disasters mentioned in the tweet. The first photo was several months, rather than one month after the atomic bombing, but is still a remarkable photo.