Students of area High Schools rally at Marjory Stoneman Douglas High School after participating in a county wide school walk out in Parkland, Florida on February 21, 2018. (RHONA WISE/AFP/Getty Images)
RHONA WISE/AFP/Getty Images
Students of area High Schools rally at Marjory Stoneman Douglas High School after participating in a county wide school walk out in Parkland, Florida on February 21, 2018. (RHONA WISE/AFP/Getty Images)
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PARKLAND, FL - FEBRUARY 20:  Tyra Heman, a senior at Marjory Stoneman Douglas High School, holds a sign that reads, 'Enough No Guns,' in front of the school where 17 people that were killed on February 14, on February 19, 2018 in Parkland, Florida. Police arrested 19-year-old former student Nikolas Cruz for killing 17 people at the high school.  (Photo by Joe Raedle/Getty Images)
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PARKLAND, FL - FEBRUARY 20: Tyra Heman, a senior at Marjory Stoneman Douglas High School, holds a sign that reads, 'Enough No Guns,' in front of the school where 17 people that were killed on February 14, on February 19, 2018 in Parkland, Florida. Police arrested 19-year-old former student Nikolas Cruz for killing 17 people at the high school. (Photo by Joe Raedle/Getty Images)
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Story highlights

Students nationwide take part in a walkout Wednesday

The 17-minute walkouts were a memorial and a protest

(CNN) —  

For the first time since a gunman turned Marjory Stoneman Douglas High School into a scene of carnage, students had their first full day of classes Wednesday.

The massacre of 17 students and faculty members added to a grim statistic: Three of the 10 deadliest mass shootings in modern US history happened within five months of one another.

In the four weeks since the Valentine’s Day shooting, the survivors have turned into activists on the national stage. Even as they grieve, they’ve demanded action on gun reform. In between congressional meetings and protests, they’ve attended memorials and funerals.

Here’s what we’ve learned since the shooting:

Here’s what we’ve learned in the aftermath of the shooting, and what’s happening next:

The students

Many of the students have confronted state and congressional lawmakers, demanding a ban on assault weapons similar to the one used to kill their friends and teachers.

Students across the country continued the fight for gun reform Wednesday by walking out of class, many of them for at least 17 minutes – one for each person killed in Parkland.

Some schools allowed students to walk out and provided additional security to ensure safety, while others forbid participation.

And on March 24, gun control activists nationwide will participate in the March for Our Lives in Washington. The event was created by Stoneman Douglas students.

A local March for Our Lives in Parkland is also planned for that day. Stoneman Douglas junior Casey Sherman, a coordinator for the local march, said many people want to get involved but may not be able to make it to Washington D.C.

The investigation

Nikolas Cruz, 19, has confessed to the shooting. He was arraigned Wednesday on 17 counts of murder and 17 counts of attempted murder. A judge entered a not guilty plea on his behalf after his defense attorney declared Cruz was standing mute – meaning he was declining to enter a plea.

Cruz remains in the Broward County Jail, where he’s segregated from other inmates. Prosecutors announced in court filings Tuesday that they will seek the death penalty. They listed several factors, including that Cruz knowingly created a risk of death for many people and that the shooting was “especially heinous, atrocious or cruel.”

His attorneys have indicated he is willing to plead guilty to avoid the death penalty. Prosecutors asked the court to put several provisions in place in the event his defense should introduce his mental health. Cruz’s defense team has said he battled with mental illness and depression after his adoptive mother died.

The case will involve a complex web of finger-pointing on who could have helped prevent the massacre, and what signs authorities missed about the confessed gunman.

The legislation

What’s new: Florida House representatives approved a bill that would raise the age for buying firearms to 21; require a three-day waiting period for purchases; and allow some school staff members to be armed on a volunteer basis.

Florida Gov. Rick Scott signed Senate Bill 7026 into law last week, the first gun control legislation in the state after the massacre.

The law, known as the Marjory Stoneman Douglas High School Public Safety Act, tightens gun control in several ways, and also allows some teachers to be armed.

The National Rifle Association opposed a provision of the law that raises the minimum age to purchase a firearm from 18 to 21. It immediately filed a federal lawsuit against Florida, saying the age mandate violates the Second and 14th Amendments to the Constitution.

A controversial part of the new Florida law allows for the arming of some teachers if the local school district and local sheriff’s department agree. A few days after that bill passed, the White House proposed providing some school personnel with “rigorous” firearms training, and backed a bill to improve criminal background checks on gun buyers. It backpedaled on the idea of increasing the minimum age to buy certain firearms – a policy President Donald Trump had said he would support.

The school district’s response

What’s new: The Broward County school board passed a 24-point resolution Tuesday, calling for Congress to ban assault weapons, require universal background checks and broaden the perimeters of school gun-free zones.

What’s next: Superintendent Robert Runcie said he wants an immediate, independent review of the social and educational history of Nikolas Cruz.

While the school district took steps to try to prevent such carnage in the future, a student who was shot five times plans to sue the school district, Broward County, and the Broward County Sheriff’s Office.

The local authorities

Much of the recent blame has fallen on the Broward County Sheriff’s Office. One of its armed deputies stayed outside the school as the massacre unfolded. And in the past decade, authorities received more than 20 calls about Cruz and his family.

Embattled Broward County Sheriff Scott Israel has rejected calls for his resignation amid accusations that his department’s incompetence failed to stop the gunman.

The sheriff’s office recently launched a website dedicated to “setting the record straight.” It says that, while deputies responded to Cruz’s home multiple times in the past, there was nothing criminal nor dangerous happening that would warrant an arrest.

CNN’s Eliott C. McLaughlin, Rosa Flores, Chuck Johnston, Kevin Conlon, Dakin Andone and Ray Sanchez contributed to this report.