Seventy per cent of curriculum planners said implementing the broader programmes of A-levels and GNVQs would be difficult or very difficult, the survey of 50 schools found.
From September 2000, students will be able to study up to five subjects in the sixth form, taking new AS - advanced subsidiary - exams after their first year, before continuing with three or more to A-level.
The reforms include more flexible GNVQs and an increase in the emphasis on key skills.
But schools are concerned that extra choice for students will increase the number of timetable options and require extra staff. Eight per cent expect timetabling to be easy.
Half said they needed extra staff to deliver the broader curriculum - on average three more full-time teachers per school.
Nearly 70 per cent of schools believe the changes will lead to bigger sixth-form classes. Only 18 per cent said they could deliver the reforms without increasing class sizes.
However, the Government announced an extra pound;35million last week to help schools implement the reforms. The amount each school will receive will depend on its size, but the extra money may allay schools' fears.
The survey also showed most schools do not plan to offer the new world-class tests when they are introduced in September next year.
Nearly 70 per cent said they intended to wait and see how the tests operate and how universities view the new qualifications.
Most schools - 76 per cent -- intend all their post-16 students to get the new key skills qualifications in communication, numeracy and information technology.