The internet is full of tips for writing a valedictory speech. YouTube also has many worthwhile samples.
wikiHow includes the following suggestions:
- Keep it in perspective.
- Avoid trying to write something that doesn’t reflect your true self.
- Be open to what the rest of the grade is thinking.
- Create an introduction.
- Thank those people who have made a difference in your life and the lives of other students.
- Add in memories.
- Acknowledge others.
- Stay out of cliche zone.
To fully unpack each of these go to: http://www.wikihow.com/Write-a-Valedictorian-Speech
The website about education (http://homeworktips.about.com/od/speechclass/qt/valedictory.htm) also provides advice around:
- Know Your Purpose
- Compile a List of Highlights
- Writing the Speech
- Use Anecdotes and Quotes
- Plan for Time
- Tips for Preparing to Speak
Finally http://grammar.yourdictionary.com/style-and-usage/writing-your-valedictorian-speech.html has general tips for Valedictory Speeches including how to begin writing it, getting feedback and involving your peers.
For an example of a recent speech given at an MPPS graduation check out http://grace2012.global2.vic.edu.au/