Well tomorrow's earth day, so think green.
You could use hte assembly to start a waste reduction campaign. Encourage kids to bring drinks in reusable nalgene or simmilar containers. Food should be carried in reusable tupperware and lunch boxes. Provide a seperate trash can for each class at all school meals and snacks. weigh each classrooms trash each day and post the results on a big poster. Give a prize for the class that makes the least waste at the end of the week. Maybe that class gets to paint a special mural at the school, or lead morning announcements for a week. Don't make the prize based on consumption!
Get them excited with a contest at the assembly. The classes can cluster in groups (one class is one group) and brainstorm ways to reduce garbage waste at school. The group with the longest list gets dismissed first from the assembly.
---Or, if your school has a lot of windows, have each class trace, cut out, and hang paper birds sillouhettes. This prevents birds from being injured by flying into the window because they don't see it.
---Or, start a compost pile, and take this opportunity to teach the kids what can be composted (napkins, fruits vegetables and grains. If you put in meat or dairy it will take a bit longer to decompose.
read http://www.compostguide.com/ to get specifics
Whatever you do good luck and have fun.