Fun fact: it’s older than most people think.
I’ve seen “English as a Second Language” in 1950s teacher handbooks, and the acronym ESL starts showing up in the 60s. For a quick peek, this Ngram shows the mid-century climb (Google Ngram Viewer: English as a Second Language) — small caveat: lots of schools now prefer “EAL”/“ML” because labels age faster than my lesson plans.