Plaid Cymru is calling on the Welsh Government to publish guidance on how GCSE papers will be marked this year. It comes after the Welsh exam board WJEC voiced concerns that the situation which led to 2000 English Language GCSE papers being remarked in Wales last summer could arise again this year.
Plaid Cymru described what happened last year as a "great mistake" saying that the fact Welsh students have sat some GCSE modules without fully understanding how they will be graded is "unacceptable".
In September, 2000 students had their papers regraded after concerns that the changes in grade boundaries midway through the year meant many had received lower grades than expected.
The Plaid Cymru Shadow Education Minister, Simon Thomas, said: "The Welsh Government has suggested that the comparators for setting the grades of Welsh students will change, but has failed to produce an explanation of these changes.
"What we need, above all, is a regulatory system that we can trust in. It is the Welsh Government's job to ensure the standards of our education system, and the government now needs to reassure the exam boards and Welsh students that last year's fiasco will not be repeated."