"I worked at school for two years" and "I left it because I was fed up with teaching" are two clauses that can be read separately. Neither of them doesn't serve as an adjunct clause modifying the verb of one another. If you want to know when you should use past perfect tense in this case, you can put the sentence this way "I had been tolarating this school for two years, but I left the job because I was fed with teaching" two pieces of messages can be linked together to give u a better idea why the person leaves his job.

Mariusz, it's the same. You can say it both ways. The two of them are correct.

The past perfect is considered bad English but they mean the same thing.