A good thesis introduction of the importance of school activities is "School activities are very important in our daily life's". From there you can then introduce what school activities are and give some examples. Then, when you are done write the recommendations and conclusions based on your research.
Why not argue the opposite position. School activities benefit only a small portion of the school's population. Concentrate on the over 3/4 of the students who don't fit into this group. Look up elitism and add that. Your essay will stand out.