Robert Hill, a consultant to Tony Blair on education, said that the Government needs to allocate more funding to the 14-19 partnerships of schools and colleges that will deliver the diplomas.
Government research shows that a typical applied course might cost between pound;1,337 and pound;1,716 per pupil, he said. Providing one day a week of training might swallow up half a pupil's annual funding.
The new diplomas will test the relationship between Government and universities, which are already blacklisting some "softer" A-levels.
Pupils will be studying in schools, colleges and training providers which have different attitudes to uniform, discipline, study leave, and more.
Writing in Achieving More Together, to be published next week, Mr Hill said that in federations of weak and strong schools, the strong will need to "saturate" weaker schools with their approach to uniforms and behaviour.
The first diplomas will start in September, with the full range available nationwide from 2013.
John Dunford, general secretary of the Association of School and College Leaders, said uniform and discipline issues could be problems in delivering the diplomas.
Some students were scared to wear their uniform on to the campus of a competing school, he said, and others would use the opportunity to make mischief.