Saying goodbye and speaking in front of assemblies: These are two of the hardest things to do in real life.
My suggestion is to try and combine the "art" of public speaking with a simple, heartfelt, yet not over-emotional message.
You don't want to be stood at the podium bawling your eyes out, you want to deliver a speech that people will remember fondly when they look back at class 10.
So I would suggest using light hearted subject matter like anecdotes of fun times, mention things you have learned, all the exciting things you have planned after your class 10 experience...
But also stick to the 'tenants' of addressing an audience. You may want to do some research into public speaking or giving speeches in general, but what I tend to do is find video of a speech I like (it could be anything, even from a movie or Marc Anthony in Julius Caesar) and try to mimic that style.