In an answer to Green Party MP Caroline Lucas, Education Minister Anne Milton said the total 2016-17 budget for 16- to 19-year-olds in England was £5.9bn.
But the Sixth Form College Association said its analysis of the detail showed only about £5.7bn had been spent.
The government says it has tackled unfair funding in post-16 education.
But the Sixth Form College Association says standalone sixth forms still lose out compared with sixth forms that are part of schools.
And leaders of both schools and colleges say they have had to cut courses and reduce student support in response to growing funding pressures.
The researchers say the shortfall amounts to a loss of £164 per student over the course of the year.
The association says a survey of its members in standalone sixth-form colleges in October suggested two-thirds had dropped subject courses as a result of funding cuts and increased costs.
While the Association of School and College Leaders, which represents secondary school head teachers, reports similar difficulties in school sixth forms.
Sixth Form Colleges Association chief executive Bill Watkin said the quality of the education was suffering.
"We urge the Department for Education to ensure that this £200m under-spend finds its way to colleges and schools in time for the beginning of the academic year in September," he said.
"Ours is the last budget in the Department for Education that should be under-spent."
And Geoff Barton, general secretary of the Association of School and Colleges, said schools were being hit across the board by rising costs "but the situation in post-16 education is even more serious because these pressures come on top of funding cutbacks in the last Parliament".
Ms Lucas, vice-chairwoman of the All Party Parliamentary Group for Sixth Form Colleges, said she found it "hard to understand why £200m of the sixth-form education budget has not reached the education front line".
"The government needs to clarify where this money has gone and ensure that it is redirected to school sixth forms and colleges as soon as possible," she said.

Gavin Rothery almost put the visitors ahead with a powerful close-range strike which was inches over.
Managerless Harriers battled gamely and went close when Elton Ngwatala's fine shot struck the post.
But from a late corner, Liam Dickinson connected with Jake Lawlor's header and volleyed Guiseley to a victory which left the hosts 14 points from safety.
The government has under-spent its sixth-form education budget by £200m this year, analysis of figures from a written ministerial answer suggests.
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Guiseley consigned bottom side Kidderminster Harriers to their seventh straight defeat in the National League.