The driver of a school bus on which a 7-year-old student was repeatedly sexually assaulted by a 6-year-old boy has been placed on leave, officials said Tuesday. At the same time, parents of other students at Boston Renaissance Charter Public School are questioning why a monitor — responsible for supervising students — was not assigned to the bus.

In an interview with the Globe on Monday, the victim’s mother said school officials told her in early November that a 6-year-old had sexually assaulted her son, and that they learned of the incidents after other students on the bus saw “activity’’ and exposed genitals.

In a letter to parents on Tuesday, school officials said the incident on the bus involved “several’’ young students.

On Monday, the mother of the victim wondered how the driver did not notice what was happening any of the 10 times officials said her son was assaulted. She said her son, who is developmentally delayed and suffers from attention-deficit hyperactivity disorder, sits at the front of the bus across from the driver.

The newspaper is not naming her to protect the identity of her son; the Globe does not name victims of sexual abuse.

The driver of the bus was placed on paid leave as a precautionary measure on Monday, Boston Public Schools officials said in a statement. District officials said previously that they did not learn about the incidents until Monday, though the mother of the victim was notified Nov. 3. Transdev, the private company that oversees the city’s school bus fleet, placed the driver on leave.

There were no monitors or attendants on the bus at the time of the incidents, Boston Public Schools spokesman Daniel O’Brien said in a statement Tuesday, a revelation that sparked calls among parents and advocates that all buses should have them.

“There should be monitors on every bus morning and afternoon,’’ said Portia Kelley, 27, of Roxbury, whose 6-year-old son is a Boston Renaissance student. “My son wants to ride the bus so bad. But I am not comfortable with him riding the bus, and incidents like these justify my feelings.’’

Another parent said she removed her then-5-year-old daughter from her bus after a fourth grader tried to force the girl and another student to kiss in September 2014. The mother said there were no monitors on that bus, and that she now takes her daughter to school.

Currently, there are monitors on eight of 21 bus routes for Boston Renaissance Charter Public School, O’Brien said, adding that monitors are meant to accommodate students with special or individualized needs, and schools can request monitors to address supervision needs on a case-by-case basis.

A bus monitor is for students with disabilities, and those are provided by Central Office at the request of schools. An attendant is hired by the school to be placed on a bus to provide additional supervision.

Of the 3,081 total school bus trips citywide each day, 1,143, or 37 percent, have monitors, O’Brien said.

District officials said it would cost $7.6 million to provide a monitor for every bus.

Speaking to reporters on Tuesday, Mayor Martin J. Walsh, who said he learned about the sexual assaults two weeks ago, called the assaults “concerning’’ and vowed to look into the parents’ request for additional bus monitors.

“The issue is the cost and how you do it appropriately,’’ he said. “Parents shouldn’t have to worry about that. It’s something we have to look at.’’

Advocates, however, say that the safety of children should be a priority.

“It’s a matter of common sense, having bus monitors or other people on a bus whose responsibility is to watch children, especially those who have special needs,’’ said Tyler Fox, an attorney in Cambridge who handles sexual assault cases.

It was also revealed Tuesday that the Boston Renaissance Charter Public School failed to alert the state Department of Elementary and Secondary Education of the sexual assaults. Charter schools are required by law to notify the state agency “in writing of all significant matters within two business days.’’

State Department of Elementary and Secondary Education spokeswoman Jacqueline Reis said school officials did not understand that they were supposed to notify the department, but that they did contact the state Department of Children and Families and the Boston Police Department. The Department of Children and Families is investigating.

“The school understands at this point that they should have notified us,’’ Reis said.

The mother of the student who was sexually assaulted said Tuesday that she has not heard from the school’s headmaster, but a school transportation official told her a bus will be available with a monitor for her son on Monday.

She said she may ride with her son to be sure he is safe. She has been taking her son to school since she learned of the attacks.

“I’m not excited about it,’’ she said. “I still have a fear of him riding the bus again.’’

The Children’s Advocacy Center of Suffolk County, an organization that works with child abuse victims and their families, also contacted her and said her son could begin counseling next week, she said. The mother was previously told he would have to wait up to three months.

Jan Ransom can be reached at jan.ransom@globe.com.