"He can't bike to school anymore, we have to drop him off."
Thompson said at 2.40pm on Tuesday a group of six youths -- not MIS pupils -- aged between 11 and 14 went onto the school grounds "uninvited".
"We try our best not to get into confrontation so we asked them in a friendly manner to please leave and they were polite back."
An MIS student later told staff that one of the youths had been asking the kids, "which way do the cool bikes go?".
The group then met an MIS student in the alleyway where one member of the group held a knife to the boy's throat before forcing him to hand over his bike.
The boy then "did the right thing" by calling police from his cell phone.
Senior Sergeant Mike Sutton said the offender was brought to the police station by family members where he had admitted the robbery.
However, the bike was yet to be recovered.
Sutton said police had treated the incident very seriously, considering its nature and the ages of those involved.
He said two or three youths were present at the time of the incident and had been spoken to by police, but only one -- in possession of the knife -- appeared to have threatened the boy.
The offender would be referred to Youth Aid.
A student who saw the situation unfold told Thompson he had tried to support his friend through the ordeal and saw a knife around 10cm long being held to the boy's throat.
A shaken-up family member had also alerted the school to the incident yesterday morning, with the same information.
An MIS staff member had seen the same group of youths around 11pm on Tuesday near the school.
The next morning graffiti was found all over the bike sheds.
"It's the biggest graffiti I've ever seen at the school," Thompson said.
Staff at the school were unsure as to whether they should publicly speak out about the incident, but said the youths were well known in the community, and people were fed up with the trouble they were causing.
The "notorious group" continuously displayed intimidating behaviour towards others.
"This is quite dangerous -- I'm frightened for our children who should be able to walk home safely without fear of this," Thompson said.