Senior school staff think their biggest task this year will be altering the way teachers work, a survey says.
The agreement takes admin tasks out of teachers' hands
Nearly six in 10 cited the government's "workload agreement" as their main concern, England's National College for School Leadership found.
The agreement puts tasks like reading the register and photocopying into the hands of classroom assistants.
"Personalising the curriculum" was the next most common priority, worrying 17% of the 838 senior staff polled.
The National Union of Teachers says the workload agreement, signed in 2003 by the other major staff organisations, could lead to the profession being undermined.
It fears classroom assistants taking over more and more of the role of qualified teachers.
However, the NCSL backs the changes.
Chief executive Geoff Southworth said: "It's really good, and perhaps not surprising, to find remodelling is such an important issue for heads.
"It's a key element of the drive to raise standards and it's also a tough business. Schools are working to tight timescales and every member of staff is affected."
"Inclusion and behaviour" ranked third in the issues of concern, at 10%.
Meanwhile, "inspection and performance measures", "extended schooling and community relations" and "closing the achievement gap" between pupils all scored 5%.
The NCSL provides training for senior staff including heads, deputy heads, heads of departments and heads of year.