Don't try to memorise your entire script. Read it over to yourself in the mirror, but practice talking like a teacher.
You know that voice. The one that is as loud as normal speak but demands attention. The "Stop talking amongst yourselves and pay attention to the lecture" voice, the one that ideally is always present. It calls your ears to listen, to keep your eyes on the speaker. It's easy: Like everyone else said, speak from your diaphragm and talk THROUGH your audience.
Always be looking up. You can write out the longest, well-worded essay ever, but you're not delivering an essay. You're delivering a -speech.- and a speech is 90% performance, which means it's 90% made up on the spot. It's less about memorisation and more about knowledge. Sure, you can memorise a lot of things. But truly knowing it means you can begin with a cursory statement and go into a mass of detail.
That's how I did my speeches in my class. I remembered the points and the direction in which they went. Remember to start with Point A and segue into point B. But don't remember that "Point A has to be worded like this!" because that's irrelevant. Don't let your body be passive, either. Don't walk around but also keep your eye moving throughout the whole class. Look at how your teacher acts. You want to be directing your speech through everyone. Start by looking at the middle of the sea of people - and they're a sea, not individuals. They are a giant mush of people you're speaking through. Demand their attention by groups. Look at the people in the middle, then to the left, the right. Catch the eye of anyone not focusing.
Most of communication is body language. Stand tall. Look proud. You're in CONTROL. Demand their attention with your stance. Don't slouch. Your hands by your side or up moving about. Your arms shouldn't be in your pockets. Nor should they be sitting on the lectern at ANY point(assuming there is one). You're on stage, and you COMMAND it.
The most important part is to just know your subject. Also, people can speak roughly 250 words in one minute. So try to write going off that. Don't talk too fast. You want to take your time. Anyone can lift a weight fast. It's easy. It gives you nothing. If you move it slowly you get more out of it. AndanywaysnobobycanfollowwhatyouaresayingifyouspeakinthevocalequivalentofdoingwhatIamrightnowanditisactuallykindofannoyingandwhatyoudonotwanttodowhendeliveringaspeechisbeannoying. But you also don't want to be annoying.
Anotherimportantthingtoremember is to keep your voice modulated, same tone and speed throughouttheentiremessagebecauseexactlylikehowitbecomes really hard to pay attention andfindmeaninginwhatIamsayingifI decide to go back and forth between typingwithoutspacesandtypingwithspaces if you're constantly talking slower and faster people will lose interest. Another thing: Don't be boring. If you're writing your speech and say "this is so boring" stop right there and choose something else. Because if it bores you, it'll bore the audience.