A few months back, I interviewed for a teaching position in a Bais Yaakov. The principal told me that she is very wary of hiring single girls, for the precise reason that they get engaged, get married and leave in middle of the year, and it wreaks havoc on the continuity of the students’ learning.
She told me that she wanted to make it part of the contract that the job is a yearlong commitment, but when she asked a Rov, he told her that the halacha is not on her side; a teacher is allowed to leave during the year. (I am not validating or contradicting the veracity of this- just repeating what the principal told me.) She maintained that it was still a matter of yashrus, and a teacher should not leave her students stranded like that.
She then told me that she had had this conversation with two single girls the previous year, both of whom had promised her that should they get married during the year, they would not leave until the end of the year. Sure enough, both of them got engaged and married within a month of each other, and they both moved to the same place. One teacher, keeping her word to the principal, drove in every day, even though it was quite a distance. The other one abandoned her promise and left in middle of the year.
The principal was naturally angry about this, and therefore asked me if, in the event I took the job and got married in middle of the year, I would commit to staying despite the difficulty. I answered in all honesty that I would hesitate to take the job in the first place under those conditions, but, if I would take it, I would absolutely 100% honor that commitment. I was brought up on the principles of yashrus and emes, lifnim mishuras hadin.