By Hannah Richardson
BBC News education reporter
The increase in teaching assistants has been linked to better results
Schools that increase spending on teaching assistants have improved results, a report suggests.
But the Training and Development Agency for Schools study says it could be the children who are not getting extra support who are benefiting the most.
This may be because it allows these pupils to concentrate on the teacher, without being distracted by others.
The government said the report showed money spent on teaching assistants had been a good investment.
The study by Dr John Brown and Professor Alma Harris at London's Institute of Education focused on changes in GCSE results in 83 schools between 2005 and 2009.
The key thing is not just to have more of them in schools but to ensure that they are well deployed in the classroom
Professor Alma Harris
It also analysed how much schools increased their spending on teaching assistants over the same period.
It comes after another report suggested that the greater level of teaching assistant support a child received, the less progress they made.
According to the TDA report, the largest increases in student attainment were found when the greatest increase in staff was in teaching assistants, not just all staff.
The authors suggest that having a teaching assistant in a class who can give additional support to those that need it, allows the teacher to focus without distraction on other students.
TDA chief executive Graham Holley said support staff were the unsung heroes of the school workforce.
"But simply increasing spending on teaching assistants isn't enough to make a difference; schools need to make sure that they are using their support staff effectively - something the TDA is working with schools to achieve," he added.
"We're also helping schools to develop frameworks to improve the quality of training for all school staff and to access funding for that training through local authorities."
Professor Alma Harris said: "The research shows that teaching assistants can make a difference to pupils' achievement if they are used effectively to support teaching and learning.
"The key thing is not just to have more of them in schools but to ensure that they are well deployed in the classroom."
A Department for Children, Schools and Families spokesman said: "This report concludes that the higher the number of teaching asistants in a school, the stronger the link with achievement and is further evidence that teaching assistants drive up pupil performance, have a positive effect on behaviour and free up teachers to do what they do best - teach.
"The investment in teaching assistants is money well spent.
"To suggest lower attainers would be better off without the additional help provided by teaching assistants is wrong - but of course it's up to schools themselves to decide how to deploy them."