Standard & Poor's has reduced the rating on bonds issued for construction of Riverhead Charter School's new $14 million building in Calverton, saying the school's charter is at "risk for renewal."

The agency reduced the revenue bond rating to BB+ from BBB- and revised the outlook on the bond from stable to negative, according to a release issued Monday. The change dropped the bonds' rating from investment grade to below investment grade.

The charter of the school, which is monitored by the state Board of Regents, is up for renewal in June 2017.

Riverhead Charter School has been in operation for nearly 15 years and currently serves 360 students in kindergarten through eighth grade. Students whose families apply to the school are selected in a lottery; their home districts pay for their education. It is one of five charter schools on Long Island.

Standard & Poor's credit analyst Debra Boyd said the charter school, in a January site visit, "fell far below authorizer benchmarks in culture, climate and family engagement, board oversight and governance." The downgrade also "reflects the school's weak maximum annual debt service coverage," she said.

Boyd added, however, that the rating agency believes "the school's good enrollment growth and very strong cash levels remain credit strengths and help to preclude a multi-notch downgrade at this time."

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The school's principal and executive director, Ray Ankrum, said in an email statement Wednesday: "The Riverhead Charter School continues to strive for excellence by providing a rigorous education in an environment where students are put first. We are confident that S&P will upgrade our next rating since parent satisfaction surveys are in the 90th percentile, we have made academic gains over the past year and have a 92.5% staff retention rate for next year, which is well above the 81% national charter school average."

A state Education Department spokeswoman said Tuesday that the downgrade in the bond rating could be included as feedback in the agency's monitoring plan.

The Board of Regents approved a three-year renewal of the school's charter in March 2014 that included a small increase in the maximum enrollment of students -- from 400 to 414.

The bond proceeds were used to construct a 50,000-square-foot building on the school's property with 18 classrooms, a cafeteria/gym, kitchen, library, faculty room and a redesigned parking lot.

Construction began in September 2013, and the building opened in January. At the opening, school officials said enrollment projections show a maximum 427 students in 2017.

The 2014 report on the school's charter renewal, done for the Regents by the Education Department's Charter School Office, gave a positive outlook.

"Overall, the school climate and culture generally support student learning, development and achievement," the report said.