Martinelli was not charged with a sexual offense because the student with whom he had an inappropriate relationship was 18 at the time.
In pleading guilty, he admitted trying to impede the investigation by telling the student to keep their relationship secret.
Two others Triton teachers and the school's former principal pleaded guilty in February to charges in the scandal that emerged in April 2012 after a female student told a substitute teacher that teachers at Triton were engaging in sexual contact with students.
The substitute reported this information to then-principal Catharine DePaul, 55, of Woodbury, who did not act and pleaded guilty to failing to report a crime, a type of disorderly person's offense.
Teachers Jeff Logandro, 32, and Daniel Michielli, 27, both of Blackwood, are to be sentenced by Brown on Friday for one count each of third-degree conspiracy to commit official misconduct. They had faced stiffer charges of criminal sexual contact and sexual assault, respectively, before agreeing to plead guilty.
They face 30 days of detention - which could be house arrest - and one to five years of probation.
One other Triton educator, former vice-principal Jernee Kollock, 39, of Williamstown, still faces a charge of official misconduct.
All five were charged in October following a summer of rumors among students and parents of the school, which has more than 1,500 students from Gloucester Township, Runnemede, and Bellmawr.
Martinelli, Logandro, and Michielli, were friends and popular with students at the school. Martinelli was a gym teacher. Logandro and Michielli taught math. Michielli also was the senior class adviser and Logandro and Martinelli coached teams at the school.
Contact Joseph Gambardello at 856-779-3844 or firstname.lastname@example.org.