On Wednesday, a 13-year-old entered the school, armed with two guns and a "kill list" - and shot dead eight fellow students and a security guard. Six other children remain in hospital.

The school is at the heart of this central Belgrade community, where streams of mourners are arriving, bearing flowers and soft toys.

Exactly 24 hours after the worst school shooting in Serbian history, pupils from the next-door college stand along the street to stand in silence and remember those killed.

"I am crushed," says final-year school student Alex Oborina, 19, beside some of the handwritten notes left on the pavement.

"We have failed as a society because this is something that should not happen to a 13-year-old. He should not be grabbing a gun and going into his school and shooting his friends."

Alex seems to echo the sentiment everyone here feels.

"This is probably the worst thing that has ever happened in my life. We need to use this as a stepping stone to rebuild. "

 I ask if the suspect's age of 13 will make it even harder for Serbian society to come to terms with this attack because he is too young to face prosecution.

"Whatever happened, even if he had been 18 or 19, justice would never have been served for those parents who lost their children."

The boy was arrested by police immediately after the shooting. His father who legally owned both guns was also detained, as was his mother.

His lawyer, Irina Borovic, says he has been interviewed by doctors and has now been taken to a Belgrade neurology and psychiatry clinic for children and youth.

The motive for the attack remains unclear.

But of the victims, seven of the eight who were killed were girls.

Further down the hill, candle wax oozes down on to the stems of white lilies resting against a thin tree.

An elderly woman makes the sign of the cross before slowly kneeling down and placing a single flower. Above her, a teddy bear is attached to the fencing of the football pitch, near the empty goal.

During the afternoon, a large crowd gathers outside the school. Most are pupils from elsewhere in Belgrade who have come here to pay their respects.

Mother-of-two Sanja Bastic has travelled here with teachers from her own children's school as a sign of solidarity.

She begins to weep as she explains how her 10-year-old son asked her husband last night what would happen if a shooting happened to them next.

"My son asked, 'how would you feel if you were having to wait outside the school and I didn't come out?'

"The kids are sometimes smarter than we are and notice things we don't."

Sanja believes her country has to look wider than the actions of just one teenager.

"It's not the kid's fault. It's our fault. This is a combination of things that are happening here. We are a great country with good people and we're welcoming but we have problems here that we need to solve drastically."

Violence both online and in real life have to be addressed urgently, she adds.

"I'm afraid this may be the beginning. We just need to stop it. We have now realised that in our country something like this can happen, and we need to take care of our kids."

A short distance from the school, at the Ministry of Education, the woman responsible for preventing violence in classrooms in Serbia tells the BBC that nothing in the suspect's school records showed any sign that he might be capable of such devastating violence.

"Official information we have about this particular case tell us that nothing suggested something like this will happen," says Milja Krivokuca.

"The most important thing for us to do now is to support everyone, stabilise the situation and then we will go back and look retrospectively to see if something could have been done differently."

At least two unions representing teachers say their members will go on strike tomorrow, demanding better protection for staff and pupils.

President Aleksandar Vucic has suggested the age of criminal liability may be lowered from 14 to 12 in the wake of the killings.

Ms Krivokuca agrees: "It is my opinion that it would be desirable to lower the age for criminal responsibility because of one important reason: It is necessary to secure maximum protection [for schools]".

Serbia should also compare its legislation with neighbours in Europe and further afield, she believes.

"We might need to see what other countries are doing where legal responsibility starts at 12, and sometimes 10 years old."

 By Nick Beake