A Montana judge apologized on Wednesday for saying a 14-year-old rape victim was "older than her chronological age" and had as much control of the situation as the teacher who raped her – remarks that prompted protests and a petition for his resignation.

District judge G Todd Baugh made the comments on Monday, while sentencing former Billings Senior High School teacher Stacey Rambold to a 15-year prison sentence, then suspending all but 31 days and giving him credit for one day already served. The sentencing came after Rambold, 54, had left a sexual offender treatment program that was part of a deal to have his prosecution deferred. The judge said he wasn't convinced that the reasons for Rambold's move were serious enough to warrant the 10-year prison term recommended by prosecutors.

Faced with backlash over his comments and the sentence that protesters considered too light, Baugh wrote an apology in a brief letter to the editor to The Billings Gazette. The newspaper provided a copy of the apology to The Associated Press.

"I'm not sure just what I was attempting to say but it did not come out correct," Baugh wrote. "What I said is demeaning of all women, not what I believe and irrelevant to the sentencing. My apologies to all my fellow citizens."

He added: "I will add an addendum to the court file to hopefully better explain the sentence."

A protest scheduled for Thursday outside Yellowstone County Courthouse will go on despite the apology, said organizer Sheena Rice, stressing that it was important for the community to show it is not going to stand for the blaming of victims.

"I'm glad he apologised, but he should have known better as a judge," Rice said. "The fact that he said it makes me think he still believes it."

A petition will be circulated at the protest, calling for Baugh's resignation. An online version had more than 8,500 signatures by Wednesday morning. If the petition and protest are not enough to force Baugh's resignation, protesters will shift to defeating him in the 2014 election, Rice said. He was first elected to the bench in 1984 and has been re-elected every six years since then, without an opponent.

Stacey Rambold was sentenced to 15 years in prison, with all but 31 days suspended. Photograph: Paul Ruhter/AP

Rambold was charged in October 2008 with three counts of sexual intercourse without consent after authorities alleged that he had an ongoing sexual relationship with Cherice Moralez, starting the previous year when she was 14. Moralez killed herself in 2010 at age 16, while the case was pending. The girl's mother, Auleia Hanlon, said in a statement to the Gazette that she no longer believes in justice after Baugh's sentence and remarks about her daughter.

"She wasn't even old enough to get a driver's license. But Judge Baugh, who never met our daughter, justified the paltry sentence saying she was older than her chronological age," Hanlon said. "I guess somehow it makes a rape more acceptable if you blame the victim, even if she was only 14."

Under state law, children younger than 16 cannot consent to sexual intercourse.

Yellowstone County attorney Scott Twito previously said he disagreed with the judge's ruling but would not appeal it. "The judge's reasons are his reasons and his reasons alone. He has broad authority under state law, given the proper criteria," Twito said on Tuesday.