Researchers in the US have been investigating just what an assessor should look for when they are evaluating teachers.

Their report indicates that a 20-minute classroom assessment done in a less subjective way than traditional in-class evaluations by principals may be the key.

A report from the University of Rochester in the US says it is possible to reliably measure classroom instruction and predict student standardised test scores in under half an hour.

Better assessment tools will provide immediate and meaningful feedback, making them important for understanding and improving instructional quality, according to educational psychologists.

For the study, researchers asked trained observers to rate the classroom instruction of 58 maths and English teachers in four high schools using an existing 15-item assessment regime.

The assessment method focused on three aspects of instruction: the engagement of students, how closely schoolwork aligns with the curriculum, and whether coursework is appropriately challenging.

In short, the researchers found that improving these three assessment areas; engagement, alignment, and rigor - is correlated with better student outcomes on standardised tests, after controlling for prior year test scores.

Study co-author Edward Deci said it shows that when students' basic psychological needs are met, learning outcomes improve.

For example, when teachers are excited about their subjects and supportive, students are more likely to be engaged, and when instructors present challenging schoolwork along with structured supports for mastering those assignments, students build a genuine sense of competence and confidence.

“The assessment captures surprisingly complex and fundamental qualities of teaching,” said researcher Diane Early.

“It's easy to use and 20 minutes is short enough for administrators to fit into the confines of their busy workday. And it's adaptable for all grades and subjects, from math and English to art and physical education.”

The study also showed that observers can use the tool reliably.

“Different observers of the same classroom came to the same conclusions,” she said.

The assessment tool was developed by the Institute for Research and Reform in Education, a non-profit organisation that uses evidence-based practices to help struggling schools.