Great question!!

So first off Braces have been around for a very long time. There is evidence of Hippocrates and Aristotle both interested in straitening teeth^1.

French Dentist Pierre Fauchard published a book in 1728 called The Surgeon Dentist which included a chapter on straightening teeth^1^2.

The first major advancements in orthodontics were from a fellow named Norman W. Kingsley (1825-1896). According to this article "A splendid thinker and skilled artisan, he introduced several innovations, including occipital traction (1879). Initially, he extracted teeth and moved the anterior teeth back into the space thus created. Later, he gave up extraction and added an inclined plane of vulcanite to his mechanism to “jump the bite.” He also experimented with cleft palate treatment, perfecting a gold obturator and an artificial vellum of soft rubber (1859), for which he was honored"

According to this article "Edward H. Angle devised the first simple classification system for malocclusions, which is still used today (Class I, Class II, and so on). His classification system was a way for dentists to describe how crooked teeth are, what way teeth are pointing, and how teeth fit together. Angle contributed significantly to the design of orthodontic appliances, incorporating many simplifications."

So the first "modern" braces appeared in the early 1900's. They were a series of loops and hooks (very different from modern braces). Here is a photo of what braces looked like from the 1900's all the way up until the last 50's/early 60's.

So in order to answer your questions. Braces existed, but they were very expensive and most people (especially during the war when resources were scarce) didn't have the money for them. It would have been rare to see your average highschool wear braces. They were usually reserved for children/teenagers/young adults of the upper class.

A little more on the history of braces: One Two Three