"Habari za" means "news of" (plural items of news of).
"Habari na" means "news and" or "news with" and those two words will only be next to each other more or less coincidentally. Pashana habari na wazazi wako! Exchange news with your parents.
yes from my experience I also would say you can use both, although "za" makes more sense because it refers to habari in class 10 not in class 9 which means its plural. And you usually want to know the news and not just one piece of news. That's why "habari za" is better.