If ISIS were really able to do this then they would have done it already.
I think one of the reasons these stories come up all the time is because that's not right.
Right now as the new(ish) high profile pupil everyone is on high alert.
It's once everyone is settled and complacent that the risk is higher, when everyone around the school is used to the security and no-one bats an eyelid at it is more when he strange car/person hanging around etc could go under the radar is a bigger risk.
I think that's why the threat to everyone is brought up all the time - keep it fresh in people's minds.