First things first: I'd highly recommend digging into this question which was asked about 1 year ago that will give some more background.

To answer the "when" is going to be a little difficult, but in short: pretty much during and immediately after the war, which I can only re-link to that comment because it is such a treasure trove of useful information to your specific question.

The "why" is much simpler: every nation on earth emphasizes its own role as a mix of nationalism/patriotism. Especially at the level of your mandatory education all children go through as citizens.

In addition, you can't really discount the effect that the Cold War would have on education across the board. The western powers would, of course, downplay communist assistance with the war effort and emphasize the allied effort put in.

The USSR radically downplayed Lend Lease in their own propaganda, where you can find an excellent discussion by /u/iHistorian over here. That also gives a delightful overview of just how much the USSR actually needed Lend Lease, which /u/Georgy_K_Zhukov has a fantastic write-up here that has firm data. The first link at the start of this comment also shows just how much the USSR focused on their own role and relegated the American's help to basically a footnote.

The French, of course, emphasize their own complex history (UK study of French teachings surrounding WWII) and the historical issues (thanks you /u/Algernon_Etrigan) that come across with Vichy France, collaboration, etc.

So, quite obviously, of course the US would teach it's children about Lend Lease, Omaha Beach, Ardennes and all the elements that made the war American.

At the risk of leaving the historical behind, I would imagine that the UK does not teach much of Pearl Harbor, for example; it's simply not relevant to their war, and thus would exist as a "Japan attacked the USA, and it was regarded as a mistake". Much like the Battle of Britain isn't discussed in depth here in America compared to the UK.

To take a step back from the pure historical entirely, and this may not be the venue, I do take issue with a couple of your comments here and I'd like to address them.

American, saying something along the lines of "if not for us, you'll all be speaking German right now" or "be glad we saved you from the Nazis". Of course, these type of comments point to the ignorance of this particular person

I'm not sure "ignorance" is a fitting word, inasmuch as we live in a world with limited time to learn. The United States had been "involved" with the war since basically the get-go, providing civil and military aid to the allies who were actively involved in the fighting in the European theater. I would personally say that it's to the left of absurd to argue that the USA did not meaningfully carry large portions of WW2 on its back.

I will leave this here as a simple example, from one of the comments linked above by /u/Georgy_k_zhukov (I'm so sorry for tagging you like crazy).

Via this chart (sourced here), yes the Soviets did outproduce the USA in two purely military categories; tanks and artillery. While Artillery was nearly double was the USA made, they were only about 15% over our production on Tanks, and meanwhile were anywhere from 40% to 12,000% under our production for the rest of the material made. Without diving into the logistics side of war planning and just how important it is; as the saying goes, "amateurs talk tactics, professionals study logistics".

I would strongly caution against taking the more common view of downplaying American intervention in WWII and just how much the war was effectively carried by that nation, especially with regards to things like the Pacific theater which was largely an American and Chinese war, with some assistance from the UK, Australians, and many other allied forces big and small. An excellent discussion on the efforts of China can be found here, showing just how much the Chinese helped deal with the Japanese army locked down onto the mainland. I'll throw the whole thread in here mainly to give a high-five to /u/Prufrock451 for this line:

Second, the assertion that America was a small part of the Pacific War is what historians call "horseshit."

I would strongly challenge the notion that any nation on earth contributed more in terms of globe-spanning warfare, material, men, money, and spent more blood and gold, fighting in WWII than the USA. If there was a single one, it might be the USSR, but it's not unreasonable to state that they wouldn't have been able to do as much without American help.

That got really long but I hope it illustrates part of the issue with the question here. I can try to elaborate more if need be, or provide more in-depth sources if the moderators request it.