Pratt, 38, faces up to life in prison when he is sentenced Aug. 31 by Judge Christine Johnson.
But defense attorney Stephen McCaughey told news reporters, "We'll try and convince the judge to impose jail and probation rather than prison. We think that's more fair."
McCaughey said Pratt pleaded guilty because he "decided to accept responsibility."
The sentencing was set out further than the usual 45 days to give the defense time to compile mitigation information and so Pratt can undergo a psychosexual evaluation.
Pratt, who is free on bail, declined to comment.
A trial scheduled to begin Monday was canceled.
A spokesman for the girl's family said they were satisfied with the outcome.
"We are extremely pleased [the victim] does not need to go through the ordeal of a trial," he said.
The 16-year-old Lone Peak High School student's relationship with Pratt began when she came to Pratt in January 2009 because she didn't like her assigned Church of Jesus Christ of Latter-day Saints seminary teacher. Pratt placed her in a different class, but the girl soon began coming to Pratt's office, instead, where they talked about religion, school and "life in general," the girl said during an October preliminary hearing.
"Brother Pratt seemed like a good friend, and I trusted him," the girl testified.
But on May 1, 2009, Pratt professed his love for the girl, which sparked a two-month sexual relationship the girl believed was a prelude to marriage, even though Pratt had a wife and three children.
The girl testified of more than 30 sex acts, mostly fondling and oral sex. But on three occasions, she said, Pratt unsuccessfully attempted sexual intercourse.
She said the trysts occurred up Provo Canyon, on the roof of the seminary building, in a ravine near her Highland home, in an empty home, inside an abandoned mine near Eureka and at a hot spring near Goshen.
The case was unusual in that the girl kept a diary of sorts by using her cell phone to record milestones in the relationship.
She recorded Pratt's avowal of affection with the phone calendar entry: "Fri. May 1, B. Pratt, Luv."
Five days later, the girl left school early, hid in the back seat of Pratt's car and he drove to Bridal Veil Falls, she said. They then went to nearby Vivian Park, where Pratt kissed her for the first time, then undressed himself and the girl and he fondled her, she testified.
She recorded that day's events in her phone calendar by writing: "Wed. May 6, Kiss."
Their last encounter was June 25, she said.
Pratt was arrested July 9 after a Utah County sheriff's detective got a fax from the state Division of Child and Family Services relaying a report of child abuse involving the girl.
The girl's testimony during the October preliminary hearing was corroborated by evidence collected by police, as well as statements of people who saw them together.
"There was a lot of evidence," prosecutor Julia Thomas said.
After that hearing, the judge dismissed six of the original 21 counts because the girl's testimony did not match dates set down in charging documents.
Pratt's attorney unsuccessfully argued that several other counts should have been dismissed, saying Pratt's authority as a teacher ended on May 29, 2009, at the end of the school year. The basis for all the charges was that Pratt was able to exercise undue influence over the girl because he held a "position of special trust."
The LDS Church terminated Pratt's employment after the criminal allegations surfaced.