Dear Jack,
I am sorry to hear that you're having trouble adjusting to your high school life. Actually, many freshmen are confused about high school life, so don't worry about it too much. I'm writing to offer you some useful advice.
First, I recommend that you sign up for extra-curricular activities, which can help you make more new friends. In this way you won't feel lonely and your school life will become colorful and interesting. Second, you should make a good schedule and focus on your studies. As a teenager, it is our responsibility to study hard.
I hope my suggestions will be suitable for you.
Yours,
Li Hua

Dear Mike,
I was worried when I heard your present situation. Therefore, I'm writing to offer you some suggestions for a healthy life, aiming to break your bad habits which are harmful to your health.
First of all, keep a balanced diet and have meals regularly, which will build up your strength. Secondly, I strongly recommend you taking some exercise. Doing exercise can make you feel energetic, thus enabling you to devote more time to your study.
I hope you can take my suggestions, build up your body and make steady progress in your studies.
Yours sincerely,
Li Hua