参考答案

One possible version:
Boys and girls:
    I wonder whether you have ever noticed that many of you don’t like to attend sports exercise for different kinds of reasons. You spend too much time on study, internet, or even computer games. Thus, you can easily get tired and often feel weaker than your parents.
    What causes it? You are lack of exercise.
    Proper exercise will not only make your bodies stronger, but also refresh your brains, which can help you study or work more efficiently. So regularly go out for exercise instead of staying at the table or before the computer screens for long hours.
    What can you do? All sorts of sport you can participate in, such as walking, cycling, running, climbing, skating, swimming, playing ball games and so on.
    Once you decide on one, you must keep on and never give up.
    Come on, boys and girls, let’s do it for a stronger body, better life, and more prosperous future.