As an Italian, devano is a rare, old-fashioned form, and I myself had to check whether it was recorded in a dictionary. So, yes, it's technically admissible but nowadays it sounds quite unusual. Presumably, your teacher aims at teaching you contemporary, usual Italian.

As an illustration, here is a graph comparing the occurrences of debbano and devano in Google Books Ngram Viewer corpus from 1800 to 2000:

Source

(Debbano itself appear to be declining: this is possibly due to a general declining use of the subjunctive mood in some registers of Italian.)

As an Italian, devano is a rare, old-fashioned form, and I myself had to check whether it was recorded in a dictionary. So, yes, it's technically admissible but nowadays it sounds quite unusual. Presumably, your teacher aims at teaching you contemporary, usual Italian.

As an illustration, here is a graph comparing the occurrences of debbano and devano in Google Books Ngram Viewer corpus from 1800 to 2000:

Source

(Debbano itself appears to be declining: this is possibly due to a general decline in the use of the subjunctive mood in some registers of Italian.)