Aspartame is not going to hurt you.
Yes, it breaks down into methanol and formeldahyde in your body, both of which are toxic in high doses, but you'd have to drink 600 cans of diet soda to get as much of either substance as there is in one orange.
the rats got cancer because they fed them so much of it that it crystallized in their stomach and THAT gave them cancer. Like eating sand.
The American Cancer Society, the American Heart Association, the National Cancer Institute, and the FDA all say it's not going to hurt you. Of course, some other people I've never heard of say otherwise... but aspartame is the most-studied substance in FDA history and has been consistently declared safe.
Aspartame rant aside, I think if they want to make school lunches healthier they need to make the lunch healthier. In high school the only "healthy" food available was warm milk and a slimy, highly unappetizing deli sandwich. Nobody's going to eat that. Other options were Arby's sandwiches, nachos, chicken nuggets, or pizza.