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Congratulations on being asked to give this speech! Not everyone gets to welcome a graduation ceremony. I'll try to help by giving you a general structure that you can use when you write it.
Start by thanking everyone for attending and for spending their time honoring the seniors who worked hard to get to the point they're at now.
Then comes your success stories. Figure out what you want your main message of this speech to be and pick your stories accordingly. Do you want the audience to feel nostalgic for everything that's about to change? Should they be smiling about new opportunities ahead? What emotion do you want to evoke and how can you do that? Find some quotes that can accompany your stories and include those as well.
Then move on to introduce everyone you need to and thank the faculty. Introduce the two seniors and congratulate them once again on their accomplishments. Then let them take the stage.
Humor is not only encouraged, but to some degree, expected. Make a joke about "senioritis" or about the students who still have to make it through more years of schooling. Poke fun at all of the ramen noodles that stereotypical college students eat. Unfortunately, I'm not a funny person but anything that makes the audience smile is worth it.
End with a bang. This is an exciting moment for a lot of people; act like there's no where else you'd rather be.
Practice, practice, practice! Have someone listen to you read your speech beforehand and have them comment on your delivery.
Most importantly, have fun. You only have one more year left yourself.
Good luck with your speech and let me know if you need any more help!