It depends, do they attend school for about 3 years in the present day or did this slight increase affected this statistic? If it did affect it, I would say no in order for it to be grammatically correct. Why? Because past participle (have attended- in this case) is used when something started in the past but it is still true or affects the present. So in case this change was temporary (from 1980 to 1990) and then everything turned back as it was before, so it is something that is still true in the present, and this increase in years of schooling didn't affect this statistic, then most definitely yes- have attended !

I would say yes. “People in developed nations have attended school for an average of three years” although I understand the sentence either way.

Thank you.

If you said “had attended” it would mean that the number has changed since then. You might use it if you wanted to compare it to today’s situation, for example.