Roger Hancock, an expert on teaching assistants at the Open University, said that only around one in 10, or about 10,000 assistants, currently has the skills to take whole classes with little or no further training.
The figures suggest that it will take time to create a workforce that can relieve teachers' workload. Dr Hancock describes the quality of advanced training for assistants as "variable".
Even if they can find the assistants, they may be costlier than expected. Unison, the public-sector union which represents support staff, this week increased pressure on employers to agree a deal to improve their pay, conditions and career development.
The union warned that schools and councils could be faced with "tens of thousands" of equal pay claims if talks break down. It is in discussions with a North-west authority, understood to be Lancashire, over what the union says are 1,600 cases of discrimination.
The claims are based on comparisons between the pay of predominantly female support staff and male-dominated jobs in other parts of the local authority.
A spokesperson for the employers said that they hoped the issue could be resolved without court action.
Employers and unions are trying to hammer out a deal on assistants. The first stage of this will give advice to schools on employment practices, training and staff appraisal and should be be finalised this month.
The second stage, expected in April, will set out a career structure for assistants.
Unions are also pushing for concessions on the controversial issue of assistants not being paid during holidays. Unison says that, even in term-time, assistants are often paid during school hours but not for meetings or training.
A TESUnison survey published in June found that fewer than half of assistants are on year-round full-time contracts and that pay can vary between pound;7,000 and pound;18,000.
Christina McAnea, Unison education officer, is determined that improved pay and conditions should be in place before assistants take on more responsibilities.
Meanwhile the Welsh Assembly is set to reject plans to introduce high-level assistants to classrooms.
Schools will instead be encouraged to take on extra technicians or administrative staff, including bursars or secretaries. The Welsh move is being backed by the National Union of Teachers, which opposes the use of assistants for whole-class teaching.
But the National Association of Schoolmasters Union of Women Teachers fears the Assembly's rejection may undermine the national agreement on workload.