I'm a bit hesitant to answer this, since it would depend entirely upon the specific teacher that was asked. Nonetheless some generalizations can be made. Following the war Japan viewed the United States quite favorably, particularly in the early fifties when their economy had begun to recover from the destruction and was quickly expanding. The prime minister during this period, Shigeru Yoshida, had two major goals: rapid rebuilding of Japan, and cooperation with the American occupation. His "Yoshida Plan" had three major points, economic growth, abstaining from any international political-strategic issues, and the provision of military bases to the United States. The plan was popular, and the population can be said to have been supportive. Following this national mood, we can draw the conclusion that, at least in an official manner, most teachers would likely have taught that the war had at best been unfortunate, Americans were friends, and the proper mindset going forward was to concentrate upon Japan's bright future.
After the war, the Japanese occupation authorities had a significant hand in restructuring the Japanese education system, with most reforms finished before the turn of the 1950s. So while I can't tell you about what was in the curricula at 1955, I can tell you that just four or so years later in 1959-1960, anti-military sentiment was high enough to cause massive protests (especially among university students) against a security treaty between the US and Japan to the point that it toppled the ruling government.
The answer to the question isn't so clear-cut, regardless of what answer would be given by your hypothetical schoolteacher. Sure, Japan attacked first, so if that's what you mean by aggressor then Japan was it, but the US placed untenable trade embargoes on Japan before that, so then you could argue that the US set up conditions that would lead to war with the intention of provoking a war. Of course, the trade embargoes were put in place to try and curb Japanese aggression in south-east asia, so you could say that the Japanese were the aggressors, but the Japanese (at least officially) were trying to establish themselves as a power in the region to drive out western influences from places that they felt naturally fell under their suzerainty, and it can't be denied that the concessions extracted from the Qing government by the western powers including the USA were excessive and exploitative. Basically, the question of "who were the aggressors?" can either mean literally "who attacked first?", or it becomes and endless chain of blame-gaming.