This is everything we know about the incident.
This is the end of the blog now, we will bring you more information as we get it.
Another student said: “We were locked in for about 40 minutes after out GCSE exam.
“All the doors were open because of the hot weather so we could hear all the sirens.”
A Canon Slade School spokesperson said: “As some of you may already be aware we went into an Invac situation (full lockdown) period 5 this afternoon.
“Following the sighting of a suspicious intruder on the school field. It was alleged that the intruder had a firearm.
“The police responded immediately and carried out extensive searches to ensure the children staff and local resident’s safety.
“Only when the police gave us permission to release the staff and children could we do so.
“The staff were then able to accompany the young people out of the building and inform waiting parents.
“As yet we do not know the outcome of the police investigation but have been reassured the area is safe. We will send further communication if we hear any more.
“I would like to thank our young people for their exemplary behaviour and the staff for their vigilance and care. Please could you speak to your children over the weekend to reassure them and we will continue to support them on Monday.”
One resident of Danesbury Road, which backs onto the back of the school, says there are no police there now.
One year eight pupil said: “We were in PE.
“Guys in my year came running down saying there are men with guns.
“We were locked inside, we all had to call our parents one by one.
“Everyone was praying, everyone was crying, no one knew what was going to happen.
“They were all telling us it was going to be fine.
“We then got let out.
“It was scary.”