Squeak, I’ve seen/heard of a number of married women who tried a career switch to teaching and failed.
There’s an advantage to maturity and life experience, but also an advantage to youthful idealism and nights free to prepare for class.
When you make employment conditional on factors not relevant to the actual job, you limit the number of candidates and lower the overall quality of teachers (Syag might argue that this is relevant to the qualifications for the job, but I disagree).
So sure, you might fill the actual positions, but with less qualified (certainly in the long run) teachers.
What do you think would happen if employment were conditional on staying the year? Would they limit their shidduchim to boys who were maskim to live there until the end of the year? Would they not take the job and stay home?
There might be some who would stay home, but I disagree with your generalization that these girls are simply unambitious. I think many are idealistic, but if teaching wasn’t an option, would go into other fields. Some options off the top of my head: computer programming, web design, therapy, medical billing, marketing.