Teenagers could miss out on sixth-form places while schools and colleges could lose funding due to confusion over the new GCSE grading system, a school leader has warned.
There is a lack of clarity over the change, according to the next general secretary of the Association of School and College Leaders (ASCL), with many still in the dark over what will be considered a "good" pass.
Under major reforms to exams, traditional A*-G grades have been axed and from this summer students will see their GCSEs graded 9-1 - with 9 the highest result.
Supporters have argued the move is necessary to allow more differentiation between students.
But Geoff Barton, who becomes the ASCL's leader on April 18, said more communication is needed.
In his first interview, Mr Barton told the Press Association: "The trouble with a new system is that if people are looking to simply match it across to the old system, it's not going to work.
"As soon as people start saying 'well, what is a good pass, is it a grade 4 or is it a grade 5?' then it becomes problematic, and schools are going to be asking the same question.
"At the moment there isn't a clarity about that, so should students be expected to get a grade 4, should they be expected to get a grade 5? Will schools be judged on a pass grade at 4 as doing well, or a grade 5?"
He said: "The confusion is caused by mixed messages from the Government and a lack of clear communication.
"On one hand, schools are going to be judged on the number of pupils who achieve a grade 5 or better in English and maths and in the EBacc (English Baccalaureate).
"On the other hand, the Government says that grade 4 is enough for pupils to progress to the next stage of their education if sixth forms and colleges decide it 'meets their requirements'.
"To add to the confusion, the Government's intention is that from 2019, pupils who don't achieve at least a grade 5 in English and maths will have to resit them in sixth-form or college."
He added: "These distinctions are unnecessarily complex and the Government has not communicated them at all well.
"We are likely to end up with a wide variation in entry requirements and uncertainty for young people.
"We are calling on the Government for greater clarity."
Mr Barton, currently head of King Edward VI School in Suffolk, said his school has a large sixth-form of 380 students, and the provisional numbers planning to join this autumn appear lower than anticipated.
"We don't understand why it is, but when we talk to a few of them, they say 'well, it might be because I'm going to get a grade 4 instead of a grade 5 and I won't be able to get in'."
This may be partly because teachers cannot be sure what pupils will need to get a grade 5, so students may consider other options to going into the sixth-form on the basis they will not get the grade.
A 10% drop in students at his school would mean £150,000 less in funding to spend on students, he said.
A GCSE C grade is currently widely considered a "good" pass. Many schools and colleges ask for this grade in their entry requirements, pupils have to retake English and maths if they do not get a C, and the English Baccalaureate measure recognises pupils that score a C or above in GCSE English, maths, science, history or geography and a language.
Exams regulator Ofqual has said broadly the same proportion of students that would currently get a C or above will achieve a grade 4 and above, while a grade 5 will equate to the top third of the marks for a current C and the bottom third of the marks for a current B grade.
A Department for Education spokeswoman said the GCSE reforms will create "gold-standard qualifications".
"We will of course continue to engage with all interested parties to ensure the changes are understood ahead of them coming into effect later this year," she said.