“Whoever designed the context of the modern classroom was certainly not thinking of the shy or quiet kids,” said Robert J. Coplan, a psychology professor and shyness expert at Carleton University, in Ottawa, Canada. With often-crowded, high-stimulation rooms and a focus on oral performance for class participation, he said, “in many ways, the modern classroom is the quiet kid’s worst nightmare.”

Susan Cain, the author of Quiet: The Power of Introverts in a World That Can’t Stop Talking, published by Random House this year, argues that such children often stop learning when they feel emotionally threatened in a class environment in which being an extrovert is considered the norm. “There is too often a tendency to see it as inferior or even pathological,” Ms. Cain said, “so teachers feel they have to turn the introvert into an extrovert.”

Take a typical class review session, in which a teacher asks rapid-fire questions and calls on students in turn. “So if a teacher asks a question and the person doesn’t answer right away,” Mr. Coplan said, “the most common thing is the teacher doesn’t have time to sit and wait, but has to go on to someone else—and in the back of their head might think that child is not as intelligent or didn’t do his homework.”

That slowness to speak can dramatically affect a student’s success in classrooms where vocal participation and group activities are critical. A 2011 study found teachers from across K-12 rated hypothetical quiet children as having the lowest academic abilities and the least intelligence, compared with hypothetical children who were talkative or typical in behavior.

Interestingly, teachers who were identified as and who rated themselves as shy agreed that quiet students would do less well academically, but did not rate them as less intelligent. As many as half of Americans are introverts, according to the Center for Applications of Psychological Type, located in Gainesville, Fla.

There’s a distinction between shyness—generally associated with fear or anxiety around social contact—and introversion, which is related to a person’s comfort with various levels of stimulation. A shy student, once he or she overcomes the fear, may turn out to be an extrovert, invigorated by being the center of attention.

By contrast, an introverted child may be perfectly comfortable speaking in class or socializing with a few friends, but “recharges her batteries” by being alone and is most energized when working or learning in an environment with less stimulation, social or otherwise, according to Mr. Coplan and Ms. Cain.

Mr. Coplan and his colleagues found differences between shy and introverted students as early as age 4: In play observations, shy children tended to hover anxiously just outside a group of unfamiliar children, while introverted children played quite happily on their own and did not attempt to approach other children. “It seems clear,” the researchers concluded, “that ’solitude’ is an insufficient criterion for characterizing children as ’socially withdrawn.’ ”

The research is mixed on when and why quiet students are academically challenged. Previous, separate studies by Mr. Coplan; fellow Carleton University psychologist Kathleen Hughes; Mary M. Reda, an associate professor at the City University of New York; and others have found that quiet and shy students often have difficulty with class grades, but that largely comes from lower levels of class participation and oral skills.

Some studies show introverted students can be better than extroverts at taking standardized tests.”Parents of extroverts have told me [those students] never actually learn to work alone, so when the time comes to take tests, … they have trouble,” said Ms. Cain, a former corporate lawyer and researcher.  On the other hand, she said, focusing too much on students’ work in a 30-to-a-room class environment doesn’t necessarily prepare students for the project-based group work more common in the workplace.

“I actually think our [science, technology, engineering and mathematics] shortages are a cultural problem as much as a pedagogical problem; the type of kid who likes to sit by himself and do math problems or science problems is not supported,” Ms. Cain argued. “Most science operations are done as teams, but scientists still have quite a bit of privacy and autonomy to their workday,” she said, noting that such environments are also hard to replicate in classrooms.