An end-of-school-year prank at a top Christchurch secondary school could have had "tragic consequences", the headmaster says.

A group of year 13 students at Christchurch Boys' High School (CBHS) chased and hurled water balloons and eggs at two year 11 students just before 1pm yesterday.

The stunt resulted in one of the boys being chased onto a road outside the school, where a vehicle ran over his foot.

Another student was assaulted, CBHS headmaster Nic Hill said.

Hill said it was lucky nobody was seriously injured.

"We are fortunate not to be facing tragic consequences," Hill said in an email to parents and caregivers yesterday.

Hill was "appalled" by the "disrespectful" behaviour.

All students were called into an assembly this morning.

Afterwards, a group fronted to Hill's office to apologise for their roles in the incidents.

Hill said he is not ruling out "serious consequences".

"The students have had fair warning," he said.

"Despite continual messages requesting an end to this type of behaviour, a core group of students have blatantly ignored this and let us down," the headmaster said.

"We have asked the boys to be respectful and decent, and I am appalled with the behaviour of a group [yesterday].

"This behaviour must stop. It does not reflect the school's values, and this is the message that will be communicated."