I've been in an assembly before and it's excruciating to put together and host, so I feel you. But I have a couple solutions to your problem.
1. Ask the teacher to choose someone else. So, if you're too busy, and this assembly hosting is getting in the way, consider asking your teacher to choose someone else, and explain why. You shouldn't be forced to host something when you don't have time.
2. Ask parents to tell the teacher to choose someone else. If you really have no time to put it together, ask your parents to tell the teacher. They will understand if you're too busy.
If you really want to stay in the assembly though, then here are some good tips to really get a good reputation with everyone.
Smile. It doesn't have to be a big toothy grin, just a small one.
Keep Eye Contact. If you look away, they'll get a bad impression of you.
Make sure you have your notes or memorize your lines. Preferably, I would say memorize your lines so they'll think you take the time to memorize, but it's still not bad if you bring notes. Bringing notes would be good if you need to study for the test tomorrow instead.
Try and act excited (happy or joyful, etc.) Be proud of yourself for doing it. Not only will you feel good, but the audience will be glad too! ^^