“At” should also be correct. Did you report it? Be careful with the word “never”. If I wanted to differentiate the gym teacher’s location from the English teacher, then I could say the English teacher works in a classroom and the first works in a gym or outside. Both work at a school. I could say the teacher is in school as opposed to on a field trip. If someone said “in a school”, I would wonder why we were stressing the location or if that were opposed to teaching online.
If you listen to the normal speech, you distinctly hear "una" escuela. If you listen to the slow speech, you distinctly hear "ena" escuela. I call this out because this was the very first time I've encountered the slow speech to be less discernible than the regular speech. Please correct.
Una ,on here sounds like Ina ! I am aware that I is close to U on the keyboard , but whoever writes the computer scripts for Duo does not do it well ! Words can merge together ,(probably because gaps are not left in between words ) and can sound wrong and are hard to understand ,especially for someone trying hard to learn . Come on , Duo give us some understandable language. I have managed to struggle through most of part two, to level five , but your mistakes have made it difficult at times. Does anyone even read the reports made by students ? Also it would be nice to be able to backtrack on lessons ,which is impossible once level five on each section has been completed.
INHO "The teacher works in a school" is incorrect as "school" is an exception in English regarding articles. Should be "The teacher works in school" although it doen't carry much sense as the "teacher" suggests that. "The teacher works in a primary (secondary, high, etc.) school" sounds right
You can find a detail discussion https://english.stackexchange.com/questions/310498/im-a-teacher-and-i-teach-at-in-school-or-at-in-a-school May be I had to use "unusual" instead "incorrect"