The problem with most "choose the correct tense" question is that tense carries meaning, and different tenses can be used with different meanings.
So in this case, both past and past perfect are possible. The meaning is slightly different (but not very different). As there is no particular need for past perfect in the rest of the phrase, most speakers would probably choose past tense (because it is the default tense for past actions).
But there are lots of other tenses that could make more sense. A future in the past "was going to tell" would seem to fit the meaning better. A lot depends on whether you understand "so" to mean "telling his students" or "doing the thing that he told his student to do". Both interpretations are possible.
So the answer that the test setter probably wants is "told" but it is not a good and unambiguous question.