"I worked as a teacher at a high school."
I did a little research and found that 勤める (つとめる) verb is used (among other meanings) as "to serve (as)" meaning that you can specify what kind of work (position) the subject was doing, contrary to 働く (はらたく) that only indicates that a job was being done, but not i you were doing a surgery or giving a lecture.
Please any native or skilled japanese learner can confirm this?
Yes, this is the right kind of feeling that this can have. There's also a noun - tsutome which can mean like a role or calling. There's also a certain level of willing commitment implied with tsutomeru. I guess like happily serving your company/customers sort of thing.
先生 here, is not being used as a title, but as a noun. For example, you can say, 「僕は先生です。」but you shouldn't say, 「私は田中先生です。」
I would have translated this as sensei toshite koukou de tsutomemashita