The old typographical superstition that it’s proper to use two spaces after a sentence should be laid to rest. Virtually every major style guide recommends a single space, and most major publishers and publications comply. If you don’t believe us, take any book off the shelf or visit any editorially scrupulous website and look closely at the spacing. Chances are good you will find no double spaces between sentences.
The two-space “rule” came about thanks to the monospaced type used by mid-20th-century typewriters. Because monospaced type (i.e., type in which every character occupies the same amount of space) has a uniform appearance, the double-space between sentences broke up the text and made it easier to read.
In computerized word processing, the only widely used monospaced font is Courier. All others are designed with proper proportion between letters, and double-spacing does not necessarily make the text more readable. In fact, having a huge chunk of space between sentences can be distracting and aesthetically offputting. Plus, double spacing takes more time.
If you have a teacher who won’t give up the double-space superstition, there’s nothing you can do but go along with it for the sake of your grade. But don’t let it become a habit outside that class.