Mat Holton, superintendent of the Chaffey Joint Union High School District, was handed the letter by Joseph who was arrested and jailed for allegedly having a sexual relationship with a student.
The girl, now 18, was a minor who attended Etiwanda High in Rancho Cucamonga when the alleged affair began.
Despite the arrest and the subsequent community fallout, Holton said morale at the school is in a "good place."
"This is a terrible event for the community," said Charles J. Uhalley, a 36-year member of Chaffey Joint Union board. "This, for us, was a tremendous sense of betrayal, sadness and hurt."
Tom Mitchell, principal of Chaffey's online high school program and a former Etiwanda High principal, has been named interim principal.
Mitchell has held group assemblies at Etiwanda High and opened lines of communication with students so healing could begin, Holton said.
"We've offered counseling to any of the students that need or want it," Holton said. "But very few have taken advantage of the benefit provided."
It has been a whirlwind week for the district and its officials that began shortly before the arrest of the nine-year administrator early Tuesday.
"On Monday, I received information that there could be something inappropriate going on between our principal, Brian Joseph, and a former student," Holton said.
Holton said he confronted Joseph on campus the same day that he learned about the allegations.
Holton said he ordered Joseph to stay away from the campus. The superintendent then drove to the San Bernardino County sheriff's station in Rancho Cucamonga to report what he had learned.
According to Holton, school staffers had no idea Joseph, 38, was allegedly sexually involved with the then-underage student.
District officials are investigating whether other employees knew about the alleged affair and didn't report their suspicions. Joseph is married with children.
Parents and students are still angered by Joseph's alleged actions.
"If this man, who is supposed to be a compass to our children, was doing this, how did it go unnoticed for so long?" asked James Donovan, who has a child attending Etiwanda High.
"It's a breach of everything that is decent. ... I understand that parents are upset and they should be," Uhalley said. "That is not us. We're here to protect children and not victimize them."
Teachers have been encouraged to address the issue and answer students' questions, Holton said.
"They were given the common facts that they passed on to their students," he said. "The students were told they could come to speak to the counselors by themselves or in a group."
The district is now focusing on how to prevent something like this from happening again.
"There's extensive training when you become a new employee with our district," Holton said. "There's also annual training that employees are required to complete every year."
Uhalley said the district's internal training process for administrators and teachers "needs to be beefed up - and we're going to do it."
"Our responsibility here is to do the right thing and I believe we did and will continue to do so," he said.
Joseph's court date has not yet been scheduled.
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