If you put 'farewell speech school' in a search engine you will find a lot of examples which you can use to make your speech. here is one to help you get started.
Write A Farewell Speech For 12th Std Students Consider As I Am In 11th Class?
A very Good evening to all my dear friends and teachers.I feel lucky to be here as i have been given this opportunity to Wish my friends.I would like to tell you about the importance of life.