For a long time I thought the double-space convention was strictly limited to school, since I had never seen a professionally printed book, magazine, or newspaper that used it. I just took a look at my bookshelf and pulled out the oldest book I own (from 1909), and it does indeed use double-spaces.

I just looked it up, and it seems like double-spacing fell out of use in the early-mid 20th century. From https://en.wikipedia.org/wiki/History_of_sentence_spacing :

Before the First World War virtually all English-language books were printed following standard typesetters' spacing rules. By the end of the Second World War most American books and an increasing proportion of English books were printed following the typewriter's English spacing approximation rules.[17] Around this time, the practice of single spacing became more prevalent. There were various circumstances which could have contributed to the change. For example, there was an increase in high-volume low-cost mass-produced printing (e.g., newspapers, pulp novels, magazines). Also, a significant innovation in the typewriter was the breaking of the typewriter "grid" in 1941.

Fun fact: HTML rendering explicitly squashes multiple spaces into one, so if you try to double-space your sentences, it won't display as such in a web browser. This sentence uses double-spaced words, and I'll bet it doesn't look that way to you.

I was taught to as well but entering the computer age then taught not to again. Apparently double space was related to the physical limitations somehow for typewriters? I dunno for sure but it is somehow anachronistic.