ABOUT 50 uniformed officers marched into Marjory Stoneman Douglas High School early on Wednesday, just one aspect of the heavy security as classes resumed for the first time since 17 students and teachers were killed by a troubled teenager with an AR-15, thrusting them into the centre of the nation’s gun debate.

The heavily armed police presence, designed to make the community feel secure, is also disturbing in itself, some students said.

“This is a picture of education in fear in this country. The NRA wants more people just like this, with that exact firearm to scare more people and sell more guns,” said David Hogg, who has become a leading voice in the students’ movement to control assault weapons. “I know one of those bullets could be shredding through me if I was misidentified as a school shooter,” David added.

Grief counsellors are on campus as well “to provide a lot of love, a lot of understanding” and help students “ease back” into their school routines, Broward Schools Superintendent Robert Runcie said. Officers with therapy dogs also stood outside.

It comes as a major US retailer who sold a gun to the Florida shooter says it will stop selling assault rifles no matter what the politicians decide.

Dick’s Sporting Goods will immediately end sales of assault-style rifles and high capacity magazines at all of its stores and ban the sale of all guns to anyone under 21 years old.

The announcement came two weeks after the school massacre in Parkland, Florida.

“When we saw what the kids were going through and the grief of the parents and the kids who were killed in Parkland, we felt we needed to do something,” Chairman and CEO Edward Stack said on Good Morning America.

Dick’s, a major gun retailer, had cut off sales of assault-style weapons at Dick’s stores following the Sandy Hook school shooting. But Dick’s owns dozens of its Field & Stream stores, where there has been no such ban in place.

In a letter released on Wednesday, Mr Stack wrote, “We support and respect the Second Amendment, and we recognise and appreciate that the vast majority of gun owners in this country are responsible, law-abiding citizens.

“But we have to help solve the problem that’s in front of us. Gun violence is an epidemic that’s taking the lives of too many people, including the brightest hope for the future of America — our kids.”

Nickolas Cruz, the gunman who killed 17 people in Florida, mostly students, had purchased a shotgun at a Dick’s store in November 2017, Mr Stack said.

“It was not the gun, nor type of gun, he used in the shooting. But it could have been. Clearly this indicates on so many levels that the systems in place are not effective to protect our kids and our citizens.”

Mr Stack on Wednesday called on elected officials to ban assault-style firearms, bump stocks and high capacity magazines and raise the minimum age to buy firearms to 21. He said universal background checks should be required, and there should be a complete universal database of those banned from buying firearms. He also called for the closure of the private sale and gun show loophole that waives the necessity of background checks.

Walmart, also a big gun seller, stopped selling AR-15 rifles and other semiautomatic weapons in 2015.

Meanwhile, at Marjory Stoneman Douglas High School, Wednesday’s class schedule started with fourth period, the same class students had been in just before the shooting had started two weeks’ previously.

School officials said the timing was planned so that students and teachers could return to the people they were with during the shooting. The building where the massacre took place remains cordoned off.

Student Casey Sherman, 17, said she thought the schedule was a good idea so that the kids can “get it over with,” and not worry about it all day.

Up late working on preparations for the March 14 national school walkout against gun violence, Casey said she wasn’t afraid to return to school, “just nervous”.

“We did go through a tragedy,” Casey said, who walked in holding hands with her boyfriend. “It was terrible but if you let it stop you ... it’s not how you go down, it’s how you get back up.”

A long line of cars circled the school and dozens of television trucks and vehicles were camped out nearby as students, parents and staff were ushered through a security cordon, past a “Welcome Eagles” banner and a walkway lined with flowers, photographs and other memorials. Some were returning despite severe gun wounds, but even those who weren’t hit by bullets spoke of emotional trauma.

Alexis Grogan, 15, wore the school’s colour — maroon — on the first day back to class, plus sneakers that said “MSD Strong, be positive, be passionate, be proud to be an eagle” and “2/14/18” in honour of those who died.

She said she felt nervous returning to school and worried it might be too soon to go on as usual without slain friends such as Luke Hoyer, 15, who sat two seats behind her in Spanish class.

Still, the support from her fellow students, and their fight to strengthen gun control laws have buoyed her spirits.

“I am so proud of how the kids at my school have been fighting because we all want change to happen and, as we see the progression, it really shows us that people do care and they do hear what we have to say,” Grogan said.