All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
You should strive to find work in a boys' school. If you cannot find any, then seek an elementary school or a mixed school where girls wear the hijab and abide by the Sharia. If you cannot find any school other than a mixed school where girls do not wear the hijab, then you should keenly lower your gaze, keep away from sources of Fitnah (temptation), try to call people to Allah, and enjoin good and forbid evil to the best of your abilities.
Shaykh Ibn ‘Uthaymeen was asked, "All the schools in my city, Idlib (in Syria), are mixed schools where girls are unveiled beyond normal, especially in my school..."
He answered, "You have to look for a school that is not like this, but if you cannot find such a school and you need to study, then you may learn and study there, and do your best in keeping away from immorality and Fitnah by lowering your gaze, guarding your tongue, not talking to women, and not walking past them." [Fataawa Noor ‘ala Ad-Darb]
Allah knows best.