Yes, brainstorming is an effective learning strategy on the high school level because it is a technique used by people even in more advanced areas of study, as well as in business. Brainstorming helps to get down all of the ideas in someone's mind and then put them together in a more coherent, rational way.
Brainstorming puts learning directly into the hands (and minds) of students. By encouraging students to open their minds and probe possibilities, teachers can make their students more engaged with the subject matter, and also identify unique teaching strategies. When a child feels like he or she is forced to learn in a narrow manner, many children mentally check out. Brainstorming is one remedy.
Yes, brain storming is an effective learning strategy on the high school level, because it teaches students to problem solve. Students need to learn more than just memorizing and writing things down on the test. Learning to brainstorm teaches children to solve problems for themsevles and think of new solutions.
I personally don't view brainstorming as a learning strategy, so I don't see how it could be effective at a high school level or any other level. Don't get me wrong, I believe brainstorming is very important, but it's better used to come up with ideas which can be used at the high school level.