With approximately 1.6 million teachers set to retire in the next decade, replenishing America’s teaching force should be a top priority. But filling classrooms with new teachers is only half the battle. Retaining them is equally important.

Numerous studies show that teachers perform best after being in the classroom for at least five years. According to a McKinsey study, 14 percent of American teachers leave after only one year, and 46 percent quit before their fifth year. In countries with the highest results on international tests, teacher turnover rates are much lower—around 3 percent.

This constant cycling in and out of new teachers is a costly phenomena. Students miss being taught by experienced educators, and schools and districts nationwide spend about $2.2 billion per year recruiting and training replacements.

Why are so many new teachers fleeing the profession after so few years in the classroom? Here are the top five reasons teacher turnover is an ongoing challenge:

5. BURNOUT: A recent U.C. Berkeley study of Los Angeles charter schools found unusually high rates of teacher turnover. At the 163 charter schools studied, teacher turnover hovered around 40 percent, compared to 15 percent at traditional public schools.

Since demands on charter school educators are seemingly boundless, including extended hours, researchers theorized, burnout is a viable explanation for the teacher exodus. “We have seen earlier results showing that working conditions are tough and challenging in charter schools," explained U.C. Berkeley’s Bruce Fuller. "Charter teachers wear many hats and have many duties and are teaching urban kids, challenging urban kids, but we were surprised by the magnitude of this effect."

4.THREAT OF LAYOFFS: In response to annual budget shortfalls, districts nationwide have sent pink slips to tens of thousands of teachers each spring for the past four years. In 2011, California sent out 30,000.

Retired teacher and author Jaime O’Neill believes this ongoing threat to job security has a destabilizing effect. As a new teacher, he wrote, you can expect your job “threatened each and every year when the annual state budget reveals once more that big cuts to education are coming, that you’ve been pink slipped until or unless there’s a last-minute reprieve. That yearly panic will cause you to wonder why you ever went into teaching in the first place, and you will surely make plans to seek other employment with each mention of just how precarious your employment is.”

3. LOW WAGES: U.S. Secretary of Education Arne Duncan recently said that teachers should earn between $60,000 and $150,000 per year. That’s a far cry from the current national average starting salary for teachers, which is $35,139.

Linda DeRegnaucourt, an accomplished high school math teacher, told CNN that after working for five years without a raise, and taking home an annual salary of $38,000, she simply cannot afford to continue doing the job she loves. DeRegnaucourt, like many other teachers, will leave the profession to pursue a more lucrative career.

2. TESTING PRESSURE: Since the No Child Left Behind Act was introduced in 2001, standardized test scores in math and reading have become the most important accountability measure used to evaluate schools.

Studies show that pressure to raise student test scores causes teachers to experience more stress and less job satisfaction. Many educators resent narrowing curriculum and stifling creativity in favor of teaching to the test.

On the National Center for Education Information’s “Profile of Teachers in the U.S. 2011," the majority of comments submitted by survey respondents were “expressions of strong opposition to the current emphasis on student testing.”

As states increasingly rely on standardized test scores to evaluate individual educators, determine teacher pay and make lay-off decisions, testing pressure will only increase.

1. POOR WORKING CONDITIONS: When the Gates foundation polled 40,000 teachers about job satisfaction, the majority agreed that supportive leadership, time for collaboration, access to high quality curriculum and resources, clean and safe buildings, and relevant professional development were even more important than higher salaries.

But working conditions in many public schools remain far from this ideal—especially forbeginning teachers, who are most likely to be assigned to the highest-need schools. Despite the added challenges they face, these teachers are often given few resources and little professional support.