The Sacred Heart Preparatory School in the city of Chiang Mai sent letters to apologize and explain what happened to several Western embassies, ministry spokesman Phongphan Yodmuangcharoen said.
"They did not intend to cause any problems," he said, adding the students did not understand the historical sensitivity of the uniforms.
The students, including one girl dressed like Adolf Hitler with a pasted on toothbrush moustache, wore the uniforms during their annual school sports day parade on Friday.
School tradition is to wear fancy dress for the parade, and to keep the costumes secret until the parade, so teachers were not aware of the Nazi uniforms, Phongphan said.
Once photos of the parade were posted on the internet they drew criticism from many places, including the Simon Wiesenthal Center in Los Angeles.
The school said it was sorry for the incident and it was caused by bad judgment on the part of some of their students, Phongphan said.