The continuous tense "has been teaching" is better here in my opinion because it carries more of a sense of a continuous 15-year period, whereas "has taught" could indicate a possibility that there might have been a gap at some point. But in those sentences as they stand, either version is fine.

Had been taught or have been taught?

The sentence:"I have taught them". Something that happened in the past, but not too long ago. "I had taught them" This sentence is further in the past. "I have been teaching them." This sentence started in the past, and is still happening.

Which is correct taught or teach?

Simple past tense and past participle of teach. Taught is the past tense of the word teach. An example of taught is for a teacher to have presented a grammar lesson on commas to her class yesterday.