Issues of discipline, the effect on the performance of existing older students and funding remain unresolved, says David Collins, principal of South Cheshire sixth-form college.
He said some colleges might feel under pressure to go along with the policy even if they believe that it will undermine their existing provision.
An Ofsted report in June awarded the college 13 grade-one ratings - the highest ever - but Mr Collins says its success is due in part to the fact that it is seen as a institution for "young adults".
It is precisely this ethos, of providing an environment where all students attend voluntarily, which he says could be undermined if colleges feel they must take under-16s part-time from schools in increasing numbers, as the Department for Education and Skills intends.
In a paper to the Association of Colleges, he said: "For the cynical, the move to a 14-19 phase could be seen as a convenient invention to endeavour to solve discipline and curriculum problems in schools."
He will be taking part in a discussion about 14 to 16-year-olds at the AoC conference in Birmingham, which starts on Tuesday.
While he supports the Tomlinson reforms for qualifications post-14, he says the Government is too preoccupied with this age group. He said: "We can run the college in an adult atmosphere.
"If we have a lot of 14 to 16-year-olds we will not necessarily be able to do that. I don't think some items have been thought through properly.
"The child protection issue is difficult. The funding has certainly not been thought through either. I might be kissing goodbye to the knighthood and the OBE but it is important to say it."
The Learning and Skills Council refused to comment.
Fento, the further education national training organisation, says the prospect of teaching under-16s produced more calls from lecturers to its helpline than any other issue.