An Alma Mater speech for a Junior-Senior prom should include references to your time at the school, what life outside of the school is like, the importance of friendship and other things you find to be valuable information in a speech. Be sure to connect your ideas and present them in a way your audience, who will mostly be between 16 and 18, will identify with.
Writing a speech can be difficult, so put some time aside for you to plan out what you want to say. An outline is always helpful when trying to write speeches, and doing research can help, even if it is your Alma Mater. Be sure to keep what you say relevant and simple; students at a prom do not want to be bored. Tell them things about life after high school, about romance and maybe how to tell "who is the one" if you are married. Say something about what prom actually means, or give a little background to the event. Talk about life philosophies or points of view that will challenge your audience. Add in some humor to keep their attention, and make the speech short. Twenty minutes should be plenty of time to say what you want to say.