Praise be to Allah.
Fasting Ramadaan is one of the pillars of Islam, and one of the most important religious duties. It is not permissible for the Muslim to take the matter lightly at all.
If there is a conflict between religious and worldly interests, the religious interests must take precedence.
Either you should try to reconcile between fasting and your work, which is easy in sha Allaah because many Muslims carry on fasting whilst working as teachers, or you should take leave from work, even if that is unpaid leave, during this month so that you will be able to fast.
See also question no. 12592.