Stephen Byers, Trade and Industry Secretary, has agreed to broaden the scope of the new European directive protecting part-timers to include temporary and agency workers.
A Scottish Executive spokeswoman said: "Education authorities will be expected to comply with European legislation but we wil have to consider the implications."
Dan Brown, Cosla's spokesman on the issue, predicted many difficulties. "Some supply staff are brought in to cover sick leave. They could work in October and not work again until January."
Jim Docherty, assistant secretary of the Scottish Secondary Teachers' Association, welcomed Mr Byers's announcement. "This will be a major benefit for young entrants to the profession and it will end the scam of authorities trying to employ people on temporary contracts."