I’m a junior in college and I have never, even in high school, been criticized for leaving only one space.

Although I was taught in typing class to leave two spaces as well, all of my English classes have only asked for one space, so I think it is becoming much more accepted, it’s more about personal preference. In a lot of publications, leaving only one space allows for more to be written in an allotted space, so that’s one reason it’s more preferred.

BTW the practice of two spaces after the end of a sentence must be a recent addition to typing lessons, not an old practice as some are stating. I’m undoubtedly older than you and I took typing in high school as well. At that time we were taught the one space rule.

I am guessing that it is less important now because of proportional fonts. The Courier font (used on typewriters) had a fixed spacing and that sometimes resulted in visual blank space (the word skiing would have a lot of blank space in the middle, for example). To emphasize the separation between sentences, two spaces were needed.

But with proportional fonts, the characters are “kerned”, that is, the spacing between the characters is adjusted to achieve a more balanced appearance. A single blank space at the end of a sentence then is all that is necessary.

If you do, in fact, use two spaces, they will be kerned, reducing their apparent length and making them appear more like one space. So I would say that two spaces is still quite acceptable. Just for jollies, I used two spaces before “So” and “Just” in the last two sentences in this paragraph.

(And yes, 2 spaces not only acceptable, but correct.)