Kimberley Asselin teaches reading to Karen Lemus, left, and Devin Hawkins-Copeland, right, in Asselin's kindergarten class at Riverside Elementary School in Fairfax County on Tuesday May 19, 2015. Asselin is a first-year teacher who says testing has tempered her enthusiasm. (Matt McClain/The Washington Post)

Kimberley Asselin sits in a rocking chair in front of her 22 kindergartners, a glistening smile across her face as she greets them for the morning. Even at 9 a.m., she is effervescent and charismatic.

Yet behind Asselin’s bright expression, her enthusiasm is fading. Asselin, 24, is days away from finishing her first year as a teacher, the career of her dreams since she was a little girl giving arithmetic lessons on a dry-erase board to her stuffed bears and dolls.

While she began the school year in Virginia’s Fairfax County full of optimism, Asselin now finds herself, as many young teachers do, questioning her future as an educator. What changed in the months between August and June? She says that an onslaught of tests that she’s required to give to her 5- and 6-year-old students has brought her down to reality.

“It’s more than a first-year teacher ever imagines,” Asselin said. “You definitely have a lot of highs and lows, and it keeps going up and down and up and down.”

Kimberley Asselin teaches her kindergarten class at Riverside Elementary School on Tuesday May 19, 2015 in Fairfax County, Va. (Matt McClain/The Washington Post)

New federal data that the Education Department released in April shows that about 10 percent of new teachers leave the profession within the first year on the job, and 17 percent leave within five years. Although far lower than earlier estimates, it still means that many young educators bail from the classroom before they gain much of a foothold. For Asselin, testing has been the biggest stressor.

The proliferation of testing in schools has become one of the most contentious topics in U.S. education. The exams can alter the course of a student’s schooling and can determine whether a teacher is promoted or fired. In Virginia, schools earn grades on state-issued report cards based on the scores students earn on mandatory ­end-of-year exams.

University of Virginia education professor Tonya R. Moon said that tests have shifted the focus in schools and enhanced scrutiny of teacher performance.

“You get this extreme focus on nothing but tests,” Moon said. “I think to some extent we have gone way overboard at the local level.”

The Fairfax County school system, one of the nation’s largest, boasts that its kindergarten students take part in coursework that exceeds the state’s standards. Unlike most states, Virginia has never adopted the Common Core State Standards, but Virginia officials say that the state’s academic standards are just as — or more — rigorous.

Asselin said that means that even the youngest students in public school are under an academic microscope, making kindergarten about far more than socialization and play time.

“Kindergarten is the new first grade,” Asselin said. “It’s not crafting or taking care of the class pet. It’s reading and writing.”

That also means that Fairfax students, including kindergartners, are tested “on a regular basis,” to ensure they are progressing through the required material.

Ellen Keyser — who has been teaching in Fairfax for 23 years, almost as long as Asselin has been alive — served as the new teacher’s mentor in a preparation program in August. She said that teaching kindergarten classes has become far more challenging in recent years. For example, students must learn how to count to 100 by fives and 10s, and they must be able to demonstrate how to tell time on a clock.

“Teaching is hard, and when I told them this is the hardest job you’ll ever love, I meant that,” Keyser said. “No matter how much you do, you never feel you get it all done as a teacher. There’s always something else to do.”

Asselin works at Riverside Elementary, a school that serves an impoverished area of Virginia’s largest county. The children in her classroom are among the county’s most racially, ethnically and culturally diverse; more than two-thirds of her class qualifies for free or reduced-price meals; more than three-quarters of the class are black or Hispanic; and 4 in 10 students are considered English-language learners.

On the bulletin board near the front entrance at Riverside, beside fliers for youth lacrosse teams and Girl Scout camp, are leaflets for homeless students seeking shelter.

Asselin said that some of her children eat their only two meals of the day at school. Others only watch Spanish-language television at home and have struggled with English. She said the additional weight of testing has at times been overwhelming.

“There is a lot more pressure than I expected, a lot more stress than I expected,” Asselin said. “I would go home and cry. I felt defeated some days. . . . You’re considered a good teacher if all your kids pass. But if they don’t, then what? Are you a bad teacher? That’s not fair considering all that’s going on in your children’s lives. . . . If a kid is starving in your classroom they are not paying attention.”

But Asselin said that she is inspired daily by her students and understands how important her role is in their lives: “For some of them, you’re the only hug they get that day.”

One morning in class, Asselin interrupted a reading lesson to bend down to tie one of her students’ shoes. She then guided a group of children line by line through a book about sea gulls, asking the students to summarize the plot of the story and identify key characters.

“We’re pushing them and pushing them and pushing them,” Asselin said. “It makes me think: ‘How long can I last in this career at this rate?’ If things are like this now, will they get better or get worse? That scares me.”