Elementary school students have been slicing and dicing owl pellets for decades. And anyone who endured high school chemistry surely remembers when the teacher created a dancing, colorful flame using a brew of alcohol and salt.

But sometimes the experiments are done in the wrong place, or they’re done without proper protective gear, or a lot of chemicals are used when a little would have sufficed. Sometimes it’s as simple as having too many students in a single laboratory.

Now, after five dozen elementary school students in Franklin provided the latest stomach-churning evidence of what can go wrong, Massachusetts health authorities are strengthening rules on school experiments, particularly those involving owl pellets. And teachers, principals, and superintendents from Boston to Springfield will undergo special science safety training in the school year that starts as soon as this week.

The state is now telling teachers and students to make a shift. Instead of assuming that a science project is safe, they should assume it could be dangerous and adopt precautions, Matyas said, such as wearing gloves and restricting projects to a single day.

The peril of pellets was first identified in 2001, when salmonella struck a school in Bayport, Minn. One spring afternoon, the Science Club at Andersen Elementary was studying pellets in the lunchroom. The Adventure Club was there, too.

When the dissection was completed, an instructor wiped off the cafeteria tables — but didn’t use a disinfectant. And the students were never told to wash their hands, even as they tucked into an afternoon snack. Nearly 40 children became sick, and four were hospitalized.