The warning from the Professional Association of Teachers comes only a week before the Scottish Examination Board releases its final batch of Higher results. The board is being replaced next April by the Scottish Qualifications Authority as the Government paves the way for curriculum and assessment reform in the fifth and sixth years of secondary school.
At its annual conference in Cheltenham, the association's Scottish executive predicted a "professional foul" if the Government continued to back internal assessment over more costly external marking. With pressure on schools to raise performance levels, heads will be under pressure to advise staff to mark up, the conference heard.
John Kelly, assistant head at St Joseph's Academy, Kilmarnock, told delegates: "Headteachers are liable to favour generous marking rather than over-strict marking of internal assessment. It is human nature."
Higher Still would be of "little value" if the new SQA certificates were not seen to be as impartial as the old Highers. Moderation of internal marking was no substitute for examination, Mr Kelly argued.
He added: "It is an indispensable requirement in any examination system that the final assessment should be external and incorruptible. Internal assessment is very useful and can provide flexibility and compatibility with effective learning, but it cannot guarantee the reliability which will make the examination valid."
Higher Still planners accepted that external marking enjoyed more public confidence, Mr Kelly said. However, internal marking was cheaper. "This is the worrying thought," he said.
He also attacked the moderation model of assessment favoured by the Scottish Vocational Education Council and now being adopted by the SEB.
Of the assessment of talk in Standard grade English, which constitutes one-third of the marks, he said: "One would like to think that an inspector would come round and assess a sample of the pupils to see if their skill in talk was indeed at the level of the teacher's award. Not so. What happens is that the moderator comes round and tests the teachers by means of a video to check that the teachers are able to assess pupils on the same levels as the examination board.
"The moderator need never see an actual pupil. He or she does not assess any pupil. Now, it does not take a genius to realise that this type of moderation is not exactly proof that the assessments are reliable."
Despite these doubts, the Scottish executive backed the Higher Still reforms as a "valiant attempt to provide a comprehensive and coherent system of assessment throughout Scotland", although it criticised the tight timetable.
Mr Kelly told delegates: "In Standard grade, many subjects are still in the process of change and 5-14 arrangements are only partially in place. The arrangements for national testing have not yet been completely sorted out and the tests are of doubtful reliability.
And yet Higher Still is supposed to be implemented on top of these other changes in the next four years. This is not a time-scale that guarantees reliability, rather the reverse."