"It's a disproportionate time to travel such a distance, with over 360 real corners."
Her school does not qualify for Targeted Rural Funding under the present system, but would be ranked about 4.5 out of 10 on the Isolation Index.
Mrs Armstrong said the new system was an improvement, but a travelling-time yardstick would be fairer.
When the discussion document was released, Education Minister Nick Smith said the index had to be based on facts, rather than debatable opinions.
Piopio principal Brian Tegg agreed that travelling time should be the benchmark, but also wanted the ministry to consider the catchment area of isolated schools.
His school, 24km southwest of Te Kuiti, has 340 students in three toll-call zones. The school's monthly toll bill, including Internet access, is usually about $1000.
Mr Tegg said other factors should be considered besides distance to towns.
Ministry spokeswoman Jessica Wilson said the aim was to find the best method of measuring isolation, and if the index was thought to be a good idea, the ministry would do more work on how it could be used.