If you are experienced, which I doubt, for you have to actually ask this question, then you would know that, hell yeah, 10 classes a day is not just excessive, but a joke. This school is going to take advantage of you, use you up, and spit you out like tobacco juice.

10 classes a day is crazy, thats 50 a week. You need to find another job ASAP. If your as experienced and qualified as you say you are, then what your suffering from is simply ignorance of the Korean job market in general. Keep looking, there are "far" better jobs out there, with a lot less work involved to boot.