Okay, so… This is something I’m known locally as being good at and have coached people in public speaking and stand up, so hopefully my advice will help here as much as it’s seemed to help others…
People are generally on your side - This is especially true when speaking at funerals but in general most people look at public speaking and think, “I could never do that,” so they immediately support someone who is, instinctively, as there is no jealousy to get in the way (ie. "I should be up there.) Yes, there are people who heckle or think it’ll be funny to see you do badly - and schools and comedy clubs tends to be where they hang out -but in the case of a school you are covering a subject which is much more interesting than the usual stuff they are used to, you are already ahead on that score so it’s not something to worry about here, kids tend to be better than most people give them credit for, especially the rowdy ones, if it’s not a usual school subject.
Slow is KEY - Not tortoise slow, but taking your time with plenty of space breeds confidence on stage in front of people… own that first moment of silence - If you brace yourself to be comfortable and confident in that first moment of silence then the crowd will be comfortable and confident in you. Being slow, clear in your speaking and comfortable between words, is the secret to keeping people interest. Double the time you usually take between every comma and full stop and make a point of taking time to enunciate longer words - you’ll find your focus on these things also makes it more comfortable to go into a sort of auto-pilot mode. NEVER try to speed up in case you are taking ‘too much of people’s time.’ If you lose you place in your notes let them wait for you, don’t rush to find your words as you’ll only get flustered. They can wait, they are a captive audience… what else can they do but wait intently for you. You’re in charge.
You’re the boss - As soon as you stand to speak it is YOUR room, no one else’s. You decide the content of what is said in that room completely and everyone will look to you for what they should think, feel or act like… It’s important not to think about even the teachers as being in charge - it’s your room, you are the speaker and what you say goes… literally.
Mistakes are your best friend - The first time I ever spoke in public I was 14 and had to give a talk on a ‘dream come true’ type trip to Disney World I received and when I got on stage in front of the whole school it was pretty daunting… I couldn’t reach the mic at all, I was so small, and it got a massive laugh and endeared me to the whole school before I even opened my mouth as I wasn’t afraid to be laughed at… You’ll find the quality of not being afraid to be laughed at in the funniest and most loved comedians. So in public speaking if there’s a mistake or something funny and it doesn’t upset you you can be assured it will be a resounding success and something that people will walk away latching on to in a positive way. I still have people talk about that speech, usually saying how funny it was. I don’t think it was a very good or funny speech, they just remember the feeling it left them with. So don’t worry about mistakes, they are a good thing if you let them be, never be afraid to be laughed at or laugh at yourself if something goes wrong and it’s only ever a positive!
Hope this all helps.