A joint sixth form involving three schools has split after 27 years.
The Sleaford joint sixth form, made up of St George's Academy, Carre's Grammar School and Kesteven and Sleaford High School, no longer exists as it did.
Instead the joint sixth form is now made up of St George's and Carre's.
Details of the split are largely unclear, but they include the high school's view that they cannot work with the academy because of the way it is run.
And it is thought that the high school wanted out of the joint status because it wants to raise entry level grades to drive up attainment.
Chairman of governors at the school, Geoff Hotchkin , said: "Now that St George's is an academy, we cannot legally enter into any formal agreement with them.
"We looked at a partnership arrangement with the two other institutions but they do not want to enter into it."
Nick Law, headteacher at Carre's Grammar School, said he and the governing body hoped the high school would feel the need to rejoin the joint sixth form.
Paul Watson, headteacher at St George's, was unavailable to comment.