John Dunford, the general secretary of the Secondary Heads Association, believes teenagers will be left to run errands and count the stock - and that "frankly they might as well be at school".
Work-related learning becomes compulsory for England's 1.2 million 14 to 16-year-olds from this term.
Schools must include it as part of the key stage 4 curriculum - in the form of work experience, vocational courses or lessons about the world of work.
At the same time, the rules on modern languages and design and technology are being relaxed, allowing pupils to drop the subjects from the age of 14.
Dr Dunford said more schools would increase the number of vocational courses available to pupils and encourage them to take applied GCSEs or national vocational qualifications.
He said that for many pupils the new requirement would be met through work placements of up to two weeks.
"A typical experience could see a pupil doing nothing more challenging than making the tea or counting the stock," he said.
"This needs considerable development, especially with small or medium-sized companies."
A spokesman for the Confederation for British Industry said: "In some instances, work experience has not been as valuable (to pupils) or as structured as it could have been."
But employers are paying increasing attention to work experience, he said.
For example, many companies are taking on fewer pupils, but making sure that those they choose are genuinely interested in them.
He added: "Employers are looking more closely at what they do - so that it's more useful for the youngster and for the company."
Dr Dunford's comments came as figures obtained by The TES revealed that the Learning and Skills Council has so far spent pound;1.4 million on recruiting and training 120 advisers to promote enterprise in schools - around pound;11,670 per recruit.
The Government is spending pound;16m over two years on the enterprise adviser service, which is designed to promote entrepreneurship among pupils in disadvantaged areas.
The advisers are working with 1,000 schools, each of which will also receive a grant of pound;1,000.