With the overabundance of convenience food available these days and the time constraints many experience, the average child's diet has more sugar and less nutritious content than is recommended. This can lead to mood swings, lack of energy, and other negative effects that impact stress levels. (This article can tell you more about stress and nutrition and how to ensure your family gets proper nutrition even when you're busy.)
Believe it or not, noise pollution from airports, heavy traffic, and other sources have been shown to cause stress that impacts kids' performance in school. (See this article for more on how to reduce stress from noise pollution.)
Lack of Preparation
Not having necessary supplies can be a very stressful experience for a child, especially one who's very young. If a child doesn't have an adequate lunch, didn't bring her signed permission slip, or doesn't have a red shirt to wear on "Red Shirt Day," for example, she may experience significant stress. Younger kids may need help with these things.
Lack of Sleep
As schedules pack up with homework, extracurriculars, family time and (hopefully) some down time each day, kids often get less sleep than they need. (How much sleep does your child need? See this resource from About.com's Guide to Pediatrics.) Operating under a sleep deficit doesn’t just mean sleepiness, it can also lead to poor cognitive functioning, lack of coordination, moodiness, and other negative effects. (Read this resource for more on the importance of sleep, and consider helping your family adopt some habits for better sleep.)