A 12-year old pupil needed hospital treatment after being attacked by an intruder at Eastleigh's Alderman Quilley School.
The attack, in the grounds of the secondary school on Tuesday, resulted in a cut above the eye.
The assailant, thought to be of school age, came from outside the school grounds and escaped on a BMX bike.
He may have been armed with a knife or other weapon, but police said it was also possible the injury was caused by a punch.
Although the attack turned out not to be serious - the wound was sealed with surgical strips - the incident has rekindled fears about the safety of children at school.
It is not known where the attacker entered the grounds, which form part of a shared campus with the adjacent Barton Peveril College.
The attack occurred near the fence on the other side of the school, next to Eastleigh College.
Headteacher, Richard Kelly, who only arrived at Quilley this term, said he was shocked that another youth could enter school premises and assault a pupil in an unprovoked manner.
"But I'm impressed by the way pupils and staff calmed the situation down," Mr Kelly said. "They brought the child straight to reception and people were there immediately, volunteering what they'd seen.
"As there was a suggestion there might have been a weapon, we called the police straight away."
The boy's parents were contacted and they took him to hospital.
On Tuesday evening, within hours of the incident, Mr Kelly discussed the security of the school's perimeter with governors and it was agreed to carry out an emergency review of boundary security over the next two weeks.
In the meantime, staff will mount perimeter patrols, particularly when children are on breaks.
Detective Constable Nikke Lewis emphasised that it was not yet known whether there had been a stabbing.
The victim was wearing glasses and police think it may be the glasses that broke his skin, perhaps when he was punched or slapped.
He did not see if there was a weapon because he was attacked from behind.
* A youth aged 15 was arrested within hours of the incident and released on police bail.
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