On a recent Saturday morning, during my composition class, 12-year-old "Katie" took time to share her opinions of some of today's top celebrities. Singer Gwen Stefani got a thoughtful furrow of the brow, heiress Paris Hilton an appalled shake of the head, actress Lindsay Lohan a wide-eyed "No." Movie star Angelina Jolie won out for her combination of glamour and intelligence. For most of the 3.5 million children in American schools born outside of the United States, familiarity with American culture indicates mastery of the English-language skills that are crucial for their success in school. But even when immigrant students grasp teenage vernacular and celebrity culture, significant language gaps often persist.

A rising eighth-grader, Katie is one of about 20 Asian-American children who I teach as a volunteer at a weekly writing class held at a Buddhist temple in a Maryland suburb of Washington, D.C. By third grade Katie had already traveled farther, physically and culturally, than most of us ever will. She arrived in the United States five years ago from a rural town in Laos, where the neighbors kept chickens and dried fish on racks in the yard and the closest thing to American culture was an occasional broadcast on local TV. But she has left it all behind—even her musical childhood name is gone, replaced outside the home with an American nickname—to join a wholly different society.

Katie and the other kids in my class have adjusted to a new country and culture with astonishing success. While their parents struggle with grammar and accent, my students are fluent in teenage idiom. Born and raised in Thailand, China, and Laos, they trade American celebrity gossip and manage polyglot personal accounts on the social networking Web sites MySpace or xanga.com. Because of this, the American adults who meet them—perhaps sometimes their teachers, too—conclude the hard work is done. The natural impulse is to identify the students who are visibly struggling with social life and language and concentrate on helping them, while assuming all is well with the others.

But this can be a mistake. Sometimes the best-adjusted immigrant student needs a lot more help than an American adult would assume. The flawless adaptation to spoken language, social life, and popular culture can camouflage important gaps, especially in vocabulary. And in a school as diverse as Katie’s, where students hail not only from Asia but from Ethiopia, Bolivia, China, and a dozen other countries, these gaps can be hard to spot. Unattended, they can make learning more difficult than it needs to be and can weaken the performance of English language learners and the schools that serve them. The problem is all the more frustrating because in many cases it may be relatively easy to fix.

Immigrant students learn the vocabulary of popular culture from television and friends almost instantly. But to succeed in science and history, for instance, they need to know words that don’t show up often in teen conversation: "Roman," "liver," "digestive system." Even the brightest and chattiest immigrant student, arriving in school during third or fourth grade, may still lack such a vocabulary by middle school. So even though Katie is a quick conversationalist in matters of popular culture, she says she often sits quietly in class, missing the lesson, afraid a teacher will inadvertently humiliate her by asking a question she can't grasp. Gloomily, and a little ungrammatically, she writes in one of our class essay samples: "I think the most difficult thing to change [on arrival in a new country] is language. …Whenever someone ask you something you always get scare, nervous because you don’t understand what they are saying."

Katie sails through middle-school math—and does reasonably well in English, where vocabulary is taught more often than assumed. But she flounders in social studies and biology. The study guide for her seventh-grade social studies final exam illustrates the problem. The curriculum covers the geography and history of Africa, Latin America, and medieval Europe. A question on the guide asks "which of four geographic factors—precipitation, longitude, altitude, and latitude—plays no role in a country's climate." The answer is intuitive, and Katie would know it immediately in Lao. She gets it right in English because of her teacher's mnemonic device: "Ms. Smith said that 'longitude is long.'"

But, in fact, Katie has no idea what the English word "climate" means. It hasn't appeared in TV programs or in conversation with friends, and she has never heard it at home. Nor does she know the meaning of "altitude," "precipitation," "longitude," or "latitude." A few dozen words like these, sprinkled through the study guide, turn the carefully prepared resource, meant to help students prepare for the final, into a hopeless crossword puzzle.

Immigrants like Katie aren't the only students who struggle with language gaps not anticipated by their teachers. Some suggest that her problem is shared by many American students from disadvantaged backgrounds who lack exposure to the type of academic vocabulary their teachers and wealthier peers hear and use regularly. The best lessons can be in vain if based on the false assumption that every student in the class is familiar with meteorological jargon. The challenge is compounded when students, afraid of embarrassment, give no indication they need help.

Even the most highly motivated immigrant parents can do little to help with a problem like this one. Few speak English as well as their children. Uniformly anxious to see their children do well in school, constantly pushing them to study and take extra-help classes like mine, they can do little but scold when their children fare poorly.

Katie understands the problem and believes she needs to solve it mostly by herself. Her peers feel the same way. Mark, a Thai child of the same age, says the most important thing a child newly arrived in America can do is spend time studying words, working with dictionaries, and listening to Americans talk to each other. But these young students also believe teachers and peers can make the difference when a classmate runs into trouble. Katie's brother Nick recalls the experience of an acquaintance, an Ethiopian girl named Gayale, who emerged as the class' star student after teachers and classmates provided after-school language tutoring.

My experience as a tutor tells me that a little bit of help can go a long way for students like Katie. Consider her success on her social studies final. I spent an hour reviewing the questions with her, and in most cases, after a few seconds explaining the terms, they began making sense. At her final exam a week later, Katie still didn’t manage the Abbasids and the Tang Dynasty as well as the Hollywood scandals and starlets, but she did find that learning the vocabulary of history and geography is not vastly harder than memorizing the celebrities. After the test, standing very straight, she said with great emphasis and pride: "I have a B."

Immigrant students visibly struggling with English may have complex problems that are hard to solve, but at least they are easy to identify. It is much harder to spot the language gaps that trouble their more fluent peers, who are in most ways adjusting well to their new country.

For today's educators, charged with the dual goals of improving the language literacy and the academic performance of the growing English-learner population, taking the time to identify language gaps among the more fluent students may seem unimportant. But for every student who overtly struggles with language, there is a student like Katie who hides the holes in her learning. These gaps can be serious if left unattended, building over time and affecting years of classroom learning. The good news is that they may be relatively easy to fix when we do find them.