President Bush's testing regime is squeezing out science, humanities, and the arts from the timetable in favour of intensive instruction in core maths and reading.
More than seven in 10 US education authorities report cutting time for at least one other subject to make room for reading and maths, according to a comprehensive new survey.
"In some, struggling students receive double periods of reading or maths or both - sometimes missing certain subjects altogether," the study notes.
The survey of 299 authorities in all 50 states was released last week by the non-partisan Washington DC-based Center on Education Policy and represents one of the first snapshots of the impact made by the White House's sweeping 2002 No Child Left Behind Act .
It offers ammunition to critics who complain that the act's onus on standardised testing forces schools to "drill and kill" - focusing on basic instruction at the expense of a richer educational experience. Critics say this is driving students to leave school: sheer boredom and lack of engagement were identified as the biggest causes of students dropping out in a recent US study.
But the act's supporters say that ensuring students master reading and maths basics is a must, as these are the foundation for all other learning.
The study found some schools view extra time for reading and maths as necessary to help low-achieving students to catch up, while others say it is "shortchanging students", "squelching creativity" or cutting activities that keep children interested.
Jack Jennings, director of the Center on Education Policy, said schools felt pressure to narrow instruction in order to hit test targets and to avoid being publicly shamed as "failing". Under the No Child Left Behind reforms, being labelled as failing can lead to escalating sanctions culminating in staff dismissals and school takeovers.
"These are all things that put real consequences behind not raising test scores," said Mr Jennings.
Michael Petrilli, until last year a key Bush official responsible for implementing the reforms, said schools could avoid narrowing the curriculum by extending their day. He cited the example of inner-city charter chain, KIPP, whose schools are open from 7.30am to 5pm.