I'm no school captain, but I consider myself pretty advanced in the area of cheezy speeches, so here are some ideas from me.
Talk about all of them coming into school in year 7, all tiny and apprehensive, maybe mention a few students specifically (in a non-mean way) bla bla bla... just a little nostalgia trip, and then say something like "It is hard to believe we've transformed into the mature etc young men and women we are today... (You can phrase it in a more inspirational way, this is just the outline)
If you're into the whole funny or comedic speeches (that also have a serious/sincere meaning), you could try an extended metaphor. For example, our own captain started a recent farewell speech with "(Insert school-name) is like a zoo", so then he had that to structure his speech around as he went through the various animals (students), trainers (teachers) etc... It worked a treat!
Say stuff like "We've made it!" all the time, or atleast once in your speech.
Beyond the content of your speech, just try to deliver it in a lively, animated and sincere manner, because that will be the most memorable aspect of any speech. Make sure it doesn't sound like a typical prepared speechm there's nothing worse than that (as you probably know).
Goodluck, I hope this has helped you a bit!
