Parents have expressed concern that Sixth Form places at one of the county's best performing colleges will no longer be available to their children if proposals to broaden the admission age range are agreed.
Beauchamp College, in Oadby, currently has an A'Level pass rate of just under 90 per cent in grades A* to C, and is rated "outstanding" by education watchdog Ofsted.
But worries have surfaced with the launch of a consultation on proposals to become an 11 to 19 college, from August 2016, admitting pupils three years earlier.
If agreed, the academy has said it will maintain overall student numbers, which currently stand at 1,734.
Principal Hugh Howe, in the consultation document, said: "The plan is to keep the number of pupils the same, but over time to rework the year groups so that they are of an equal size.
"It is proposed that by 2020 all year groups (7 -11) will consist of 300 pupils."
But parents such as Andrew Blain, professor of observational astronomy at the University of Leicester, are concerned by the prospect.
He said: "I'm an Oadby resident who, as many others, moved there for the schools.
"The proposed changes will remove about 40 per cent of sixth form places that Beauchamp College currently provides, predominantly for Oadby residents.
"As Beauchamp is the star in Leicestershire's education system, this change will be to the great detriment of many local residents, who will have to seek private schooling or less-august state provision."
The eight-week consultation coincides with simultaneous exercises by Gartree High and Manor High academies, both in Oadby, which propose to extend their school leaving ages from 14 to 16.
Supporters of the changes believe moving to a two-tier primary/ secondary system will help drive up standards and improve choice for parents.
However, it will mean changes to catchment areas, admission priorities and entitlement to free school transport.
Prof Blain said: "My children, aged eight and 10 in Year groups 4 and 5 at Brocks Hill Primary School, are right in the firing line for the changes.
"So I've paid attention to this sudden rash of consultation papers.
"However, this week's proposals are inconsistent, appear very rushed, are full of education jargon but lacking in content, and the motivation for them is not clear."
He added: "Moreover, there is no clear route to Beauchamp's excellent sixth form for pupils who become enrolled in the now-proposing-to-be 11 to 16 Gartree and Manor schools.
"Reducing the number of transitions may be excellent news for those pupils who get to sixth form, but not for the rest."
"My understanding is that Beauchamp must have been forced into this move by the middle schools, and that the proposed changes will entail massive and entirely unnecessary disruption."
Manor High School headteacher Liam Powell, said: "Parents can be reassured that there will still be a broad range of sixth form options and choices available to their children."
Kath Kelly, headteacher at Beauchamp, told the Mercury that Manor High School was the driving force behind the changes, forcing the other schools to react.
"We have had to adapt to protect our interests and those of our pupils," she said.
"However, it does bring with it some potentially very exciting opportunities, particularly a new learning partnership with primary schools which is highly innovative and we believe will dramatically improve pupils' learning and educational development."
Mr Powell said: "I can't really comment.
"What I can say is that at Manor High we will do what's best for our children in providing a quality education."
The proposals, if adopted, must be approved by the Secretary of State for Education, and Leicestershire County Council. The consultations close on February 27.
www.beauchamp.org.uk
www.gartree.leics.sch.uk
www.manorhigh.leics.sch.uk
