High school in the US can be terrible. It starts too early, earlier than most teenagers want to wake up. Students are more likely to skip school than they are elsewhere in the developed world. About half of students say "bored" best describes how they feel during the day.

That might be why academic achievement stalls out: reading and math scores in elementary school have been climbing for American students in fourth and eighth grade. For high school, they've been flat for four decades.

"Most American kids are in schools that are very easy," says Laurence Steinberg, a professor of psychology at Temple University. "We know that because that's what kids themselves tell us in these big surveys."

Steinberg recently wrote a book on the psychology of adolescents, Age of Opportunity, that includes some of his criticisms of American high school. In an interview, he suggested three ideas to make high school better — more in line with adolescent brains, more likely to improve academic achievement, and more likely to keep kids out of trouble.

More exercise, and not just for athletes

Education research is focusing less on kids' intelligence and more on their skills, such as the ability to persist in the face of setbacks or resist temptation. Those skills are more likely to predict success in school and in life than intelligence alone.

And one way to cultivate them is physical activity, Steinberg says. Aerobic exercise benefits many parts of the brain and has other benefits as well.

But PE classes are an endangered species in high school. In 1991, 42 percent of high school students told the Centers for Disease Control and Prevention that they took gym class every day. In 2012, just 29 percent did.

Students might be making up for some of that with sports — 54 percent played on a team. But "physical education is a thing that's been more or less eliminated from a lot of high schools to the point where it's really only the athletic stars who get to exercise every day," Steinberg says.

Start school later, and keep students there later

Pediatricians endorse a later school start time because of the way adolescent brains work. Steinberg argues for it for a different reason: starting school later would mean that students stay in school later on weekday afternoons. That keeps them busy during the time of the day when, he says, teenagers are most likely to engage in risky behavior because they're without adult supervision.

It's easier to change a teenager's environment than to change his or her mind, Steinberg says. (One example: the rising price of cigarettes was a major reason for the decline in teen smoking.) He predicts rates of teen drug and alcohol use, smoking, and pregnancy would fall even more if kids were busy in the afternoon.

"You probably would make more inroads if we tried to change the context in which kids live, rather than trying to change kids," he says.

Make students work harder in class

Steinberg is a supporter of a national curriculum and national standards, saying he thinks that higher, more uniform standards would challenge more students and create more equal expectations.

The Common Core state standards are a step toward uniform national standards — 43 states will have the same expectations for what students should know and be able to do in reading and math — but they don't impose a national curriculum.

Students simply aren't pushed enough academically, he says: just one in six high school students says he's ever taken a challenging course. One idea he proposes is that more states create exit exams that are required to earn a diploma. Twenty-four states require those exams, and there's little evidence that they have been effective at raising achievement. But Steinberg argues that they at least hold all students to the same standard.

"Kids aren't being challenged," he says. "Very little is being demanded of them. Schools aren't taking advantage of opportunities."