Crossland high school principal Charles Thomas, seen in 2011 sharing a laugh with a graduating senior as he signed yearbooks, was heralded for turning around one of the county’s most troubled schools. (Mark Gail/The Washington Post)

When Charles Thomas took the reins of Crossland High School nine years ago, it was one of the worst schools in Prince George’s County. Test scores were low and disciplinary problems were among the highest in the county.

When the longtime principal retired last week — giving just two days’ notice — he left a transformed school. Crossland has had dramatic gains in state test scores, soaring participation in Advanced Placement courses and enormous growth in students who aspire to go to college.

“When he first got there, the school was dangerous and out of control and the level of rigor in the classroom was questionable,” said School Board member Edward Burroughs III (District 8), who graduated from Crossland in 2010. “Today, Crossland has been recognized nationally for the gains that they’ve made.”

Thomas said he raised standards for students and teachers to improve student achievement and “change the culture” of the Temple Hills school. He told students that profanity and disruptions in class would not be allowed; he told staff that high absenteeism and low performance would not be tolerated.

More than half of the teachers left Crossland in his first two years, he said, and they were “replaced with someone who was better.”

In another brazen move, Thomas collected the names of all ninth-graders who were 16 or older with a grade-point average of less than 1.0, or a D. Of the 800 ninth-graders, 330 students — some as old as 19 — fit the profile.

He called them to his office and told them their performance was unacceptable. Thomas gave them one academic quarter to raise their GPA to at least a 1.0 or he would recommend that they either be withdrawn from school or transferred to the evening school program.

At the end of the next quarter, Thomas said, all but 50 of the students had improved their grades.

“That proved to me that all the kids needed was someone to tell them: ‘This is not allowed anymore. You are not going to sit here like this,’ ” Thomas said last week. “That has probably done more to raise the academic performance of students who have done poorly than anything else. That’s what parents are supposed to do. Look at grades, and if they are failing, tell them this has to stop. In absence of that, we did it ourselves.”

Thomas’s leadership motivated Patricia Jackson, who doesn’t have a child attending Crossland, to become involved in the school’s parent-teacher-student association. She said she believed in Thomas’s vision for the school and its students.

“He was able to put Crossland on the national agenda,” Jackson said.

Thomas expanded the AP offerings, started an International Baccalaureate program and encouraged students to apply for college. Students took seven AP tests at the school in 1999. Five years after Thomas’s arrival, students took nearly 700 AP tests, though many did not score high enough to receive college credit. Yet he said that 90 percent of his graduating seniors apply for college.

The school was featured two years ago on the U.S. Department of Education’s “Doing What Works” Web site, which highlights best practices in education.

Thomas’s sudden departure shocked students, teachers and elected officials. Interim Schools Superintendent Alvin Crawley sent a letter to the school staff, students and parents on Tuesday, announcing that Thomas planned to retire, effective Friday.

“This comes as a surprise to all of us, but we wish Mr. Thomas the best in his future endeavors,” Crawley wrote. On Friday, he named William Kitchings, the school’s evening school principal and former vice principal, to serve as the interim principal.

Briant Coleman, a spokesman for the school system, said Thomas did not give schools officials a reason for his sudden retirement.

In an interview, Thomas said the reason for leaving is simple: He was burned out.

“I had a moment a couple of weeks ago; I had a particularly difficult day and it was about 10 o’clock at night and the thought occurred to me that I was really, really tired and I didn’t want to do it anymore,” Thomas said. “I believe if you don’t want to show up and give 100 percent, then you need to get another job. My kids deserve the best I can give.”

Thomas, who said he’d only taken three sick days in his 17 years working for the Prince George’s County school system, said he departs Crossland feeling like “I have done my part. I look at my body of work, and I know it’s a better school.”