The principal of Denver’s second-largest school, South High, has said she won’t return this fall. In a letter to families, Jen Hanson cited “personal health concerns” as the reason for her departure.

“It greatly saddens me to write this,” Hanson said in the letter, dated June 18. “A strong school is never about the leader but the staff and students inside who make it thrive, and that is South.”

Denver Public Schools has named Bobby Thomas the interim principal for the 2018-19 school year. Thomas has been principal of a small alternative high school in southwest Denver called Summit Academy for six years, according to a separate letter from the district.

The letter says the district will work with the South community to choose a permanent principal for the 2019-20 school year.

South has been on an upward trajectory for the 2½ years Hanson has been at the helm. The letter lists several bright spots, including a rising graduation rate, the second-highest college matriculation rate in the district, and being named a “School of Opportunity” by the National Education Policy Center at the University of Colorado – an accomplishment that netted South some positive press in the Washington Post.

The award was based on South’s success in educating all students, regardless of their background. South is a district “newcomer center” for refugee and immigrant students from more than 50 countries. A book published last year by Denver journalist Helen Thorpe follows the lives of 22 immigrant teenagers there. In 2016, Nobel laureate Malala Yousafzai made a surprise appearance at the school.

Almost 70 percent of the 1,600 students at South this past year were students of color, and more than half were from low-income families. Hanson’s letter notes that the number of students of color taking college-level classes at South increased from 72 in 2016 to 592 in 2018, one of the reasons cited by researchers in naming it a “School of Opportunity.”

In January 2017, shortly after President Trump announced a travel ban on citizens from seven Muslim-majority countries and temporarily suspended the U.S. refugee program, South invited local journalists to speak to a group of students in the library. Seated in a semi-circle, the students talked about how South was a safe and welcoming place.

“Even if you are a minority student or a student who’s being targeted by politicians or told you don’t have a right to be here, we want you here at South,” said then-senior Cherokee Ronolo-Valdez, who was born and raised in Denver.

In her letter, Hanson said she knows South will continue to distinguish itself locally and nationally. “South is truly the epitome of what a public school can and should be,” she said.

The district’s letter says interim principal Thomas has family ties to South: his wife, mother-in-law, and father-in-law are alumni of the school. It also points to Thomas’s track record, noting that he oversaw the improvement of Summit Academy from a low-rated school to a high-rated one. (The district’s school ratings are largely based on test scores.)

Summit assistant principal Juan Osorio will take over as principal there, district officials said.

The letter says South families should expect more information in the fall about the process of choosing a permanent principal. The district is also still searching for a permanent principal for another of its high-profile schools, Manual High School in northeast Denver.