"We are concerned that the school personnel interviewed were not randomly selected from among key individuals but rather were selected by" schools chief Kevin Maxwell, said the letter signed by Dels. Jay Walker and Geraldine Valentino-Smith, both Democrats.
The lawmakers asked state Superintendent of Schools Karen B. Salmon to conduct an "in-depth audit" and records analysis to determine whether there is evidence of grade manipulation.
Maxwell has denied the allegations, dismissing them as a politically motivated attempt to undermine the system's recent gains. He produced the results of the earlier state investigation, which found no evidence of fraud, as proof there was no systemic effort to manipulate graduation and promotion rates.
But board of education member Edward Burroughs, the de facto leader of a group of young dissidents on the panel, has said they can provide whistleblowers and documentation that proves wrongdoing. No such evidence has been made public so far.
The possible scandal could spell trouble for Prince George's County Executive Rushern L. Baker III (D), who chose Maxwell to overhaul the county school system and recently renewed his contract. On Wednesday, Baker announced that he is running for governor. He touts among his achievements launching revisions that have improved academic performance and graduation rates.
Maxwell showed his frustration at a packed school board meeting in Upper Marlboro on Thursday night, calling the allegations a "personal attack" against every school employee.
"There has been no systemic effort ordered by me or others acting on my behalf to promote students in order to inflate graduation rates," Maxwell said, adding that he and his team are open to resolving concerns "that are brought to us" but will not accept a "complete mischaracterization" of county schools, students and staff.
The grass-roots group Progressive Maryland held a demonstration before the meeting, demanding that the allegations be investigated.
The rally drew Bob Ross, president of the Prince George's County chapter of the NAACP, and former national NAACP chief Benjamin Jealous, a civil rights leader from Baltimore who also is running for governor as a Democrat. "There is a crisis of confidence in this county," Jealous said.
About 50 protesters chanted "save our schools" and "educate, then graduate," then walked into the board meeting behind Burroughs, singing "We Shall Overcome." Inside, every seat in the auditorium was filled, and the atmosphere was tense.
The list of public speakers was capped at 15 people and included Tracie Miller, principal of Gwynn Park High School, who stood at the microphone with other principals crowded behind her.
"We as high school principals are extremely offended," Miller said, reading a prepared statement. "Yes, there is pressure to ensure that students graduate, but shouldn't there be?"
Walker said in an interview that he doesn't believe in attacking the school system but that he is troubled by the possibility that students graduated without having met state requirements. He said Maxwell canceled a meeting that had been planned with delegation members, but school officials said later that the delegation was given a new meeting date.
"I'm disturbed and upset," Walker said. "Let's get to the bottom of this."
Donna St. George contributed to this report.