Abba-
Of course a morah is not a slave. you are also correct that if a sub is found, then the morah who left does not have to pay the school back. But every single morah signs a contract in September that she will work that year and be paid X amount in total– it is not a “freelance” job by the hour, but a commitment to stay the year. She is not a Poel that gets paid by the hour and can leave whenever. If you really believed that, then shouldn’t any teacher be allowed to quit at any time for any reason? Of course not, because a teacher is hired to work for the whole year, and is thus a kablan. see a similar case here: http://businesshalacha.com/en/newsletter/finish-job
Now, you could argue that a shidduch is a good enough reason for a teacher to break her contract, but I dont see how a teacher isnt a kablan.