UPDATED 2.43PM: A bus crash which has left 13 high school students requiring hospital treatment has been described as a nightmare scenario by the school's principal.

The bus full of Howick College students was on its way to an overnight camp when it crashed near Thames this morning.

The injured Year 10 students have received treatment at Thames Hospital, while the rest are returning to Auckland.

Howick College principal Iva Ropati is offering his sympathy to the families.

He says learning of the accident was extremely devastating.

Hospital manager Jacquie Mitchell says it was unusual to have so many patients in the emergency department in Thames.

But she says they they had an incident management team in place within minutes and staff coped well, while the students who showed great maturity and calmness.