MONROE, Conn. -- Classes resumed Thursday for the students of the Newtown, Connecticut, school where a gunman last month burst in and killed 20 children and six adults before killing himself in the second largest school shooting in the U.S. history.

With their school still being treated as a crime scene, the more than 400 students of Sandy Hook Elementary School attended classes in a neighbouring town.

Law enforcement officers guarding the new school called it "the safest school in America."

The school district said parents who wanted to be close to their children were welcome to visit and stay in classrooms or an auditorium throughout the day.

Newtown Superintendent Janet Robinson said officials would do their best to make the students feel at ease.

"We will be doing a normal day," she said.

The gunman, Adam Lanza, killed his mother at their Newtown home before driving to the school. He had no known connection to the school, and police haven't released any details about a motive.

One father, Vinny Alvarez, said he wasn't worried about his 8-year-old daughter going back to class. He said she got to pick out a stuffed animal to take home from the school library.

"The fear kind of kicks back in a little bit, but we're very excited for her and we got to see many, many kids today," Alvarez said. "The atmosphere was very cheerful."

Alvarez took a moment at the open house to thank his third-grade daughter's teacher, Courtney Martin, who protected the class from a rampaging gunman by locking her classroom door and keeping the children in a corner.

"Everybody there thanked her in their own way," he said.

On Wednesday, the students and their families were welcomed at an open house at their new school, which was renamed as the Sandy Hook Elementary School. Students received gift boxes with toys inside and shared joyful reunions with teachers.

Teams of workers, many of them volunteers, prepared the school and even raised bathroom floors so the smaller elementary school students can reach the toilets. The students' backpacks and other belongings that were left behind following the shooting were taken to the new school to make them feel at home.

Students found the same chairs and desks, when possible. Their classroom walls were painted the same colours and hung with the same pictures. Other details, such as the location of bookshelves and cubby holes, were replicated as much as possible.

Several signs welcoming the Sandy Hook students to their new school were posted along the road leading to it in a rural, mostly residential neighbourhood. One said "Welcome Sandy Hook Elementary Kids," while a similar sign added "You are in our prayers."