By Christopher Paslay

The Philadelphia School District has earned the right to celebrate. In 2010, more than 50 percent of the children in its schools met state standards in both reading and math. This is the eighth straight year in which the district has made gains in these subjects.

Success, of course, is relative. Critics of city schools point out that there's still a long way to go before all children are reading and doing math at grade level.

Although I agree that our public school system has plenty of room for improvement, it's misleading to suggest that all students can one day perform "at grade level" in all subjects.

Grade level is an arbitrary term. How it's defined makes all the difference in terms of what we perceive as success or failure.

If you define grade level according to what a student of average academic ability should be able to learn by a given point in his or her schooling, you're ensuring that about half of all students will not be at grade level. That's because average, by definition, means that about half the students are above it and half are below.

Interestingly, the Philadelphia public schools have been hovering around this middle point on state tests for years.

In his book Real Education, the noted social scientist Charles Murray wrote of two truths: One, ability levels vary; two, half the children are below average.

"In large groups of children, academic achievement is tied to academic ability," Murray wrote. "No pedagogical strategy, no improvement in teacher training, no increase in homework, no reduction in class size can break that connection."

Performing at an 11th-grade level as defined by the state of Pennsylvania means, in math, handling advanced algebra and geometry, and, in reading, analyzing complex literature. This is an impossibility for some children. And it's humiliating and unfair to force all students down the same path.

I say this as a dedicated 11th-grade English teacher who personally helped raise Swenson High School's standardized reading scores by 15 points in 2010.

While many people have a hard time accepting that ability levels vary in academics, not many will argue about the same principle in athletics. Imagine this scenario: 500 11th graders run a 5-kilometer race as part of their school's physical-fitness curriculum. At the end of the race, the average time of the runners works out to 20 minutes.

The school's athletic director, under pressure from the state to improve the running performance of students at his school, sets a 5K time of 20 minutes as the new benchmark for what's considered running "at grade level." The athletic director declares that by the end of the school year, all 11th graders must be able to run a 5K in 20 minutes or better, or else the school's cross-country coaches will be fired or forced to transfer.

Concerned about their jobs, the coaches step up their efforts to get all kids running the distance in at least 20 minutes. They begin forcing the 11th graders to do double workouts during the week, and they institute Saturday practices to increase their total mileage. A month later, a second 5K race is run, and 241 kids take more than 20 minutes to finish.

The frustrated coaches might explain - and few would disagree - that for an 11th grader to run a 20-minute 5K, he must have a level of ability that's not universal. He must be born with a strong cardiovascular system that can circulate a high volume of oxygen through his blood, and his leg muscles must contain ample red muscle fiber.

Of course, despite varying ability levels in both academics and athletics, neither coaches nor teachers should give up on groups of children. Educators must continue to set high expectations and challenge all students. But they should not harbor unrealistic ideas about the possibilities.

Every child has strengths and weaknesses. Public schools must work hard to see that all children reach their potential, but this potential should not be limited to core academic subjects. Learning is much broader than that.

Children who score below average in English and math might be excellent musicians, athletes, carpenters, or electricians. They might be wizards in the culinary arts. Or they might be able to repair a smashed fender or fix a fuel injector while blindfolded and standing on one leg.

Schools must understand this reality and work hard to help all students of all skill sets and ability levels achieve all of their dreams.