Principals say a shortage of teachers in Canterbury appears to be subject-related.

Schools nationwide are reaching crisis point with 370 teacher vacancies still left to fill before the end of the month.

Darfield High School Principal James Morris said some subjects are harder to fill.

"Technology is hard to find teachers for, and the ones that have been around for a while now, Maths and Science and also English as well."

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And while Hillmorton High School principal Ann Brokenshire has had three applicants for a maths -science position since advertising on Monday - it's slim pickings.

"Two of them are not yet registered to teach in New Zealand. The third applicant is already registered but has been living overseas for a while so is not back here yet."

On the other hand, Brokenshire said she was inundated with applicants when she advertised for a special needs teacher.

There are concerns that some schools may be forced to send students home if they cannot find enough teachers.

Other schools are considering upping classroom sizes.