Background checks were being carried out on teachers at Bell Baxter High
An investigation has been launched after the personal details of dozens of teachers went missing in transit.
Information regarding about 50 teachers at Bell Baxter High School in Cupar, Fife, were sent to Disclosure Scotland, which carries out background checks.
But Disclosure Scotland said it only received three of the forms. The location of the rest is unknown.
Teaching union EIS said it wants assurances that the documents have not fallen into the wrong hands.
Spokesman David Farmer told the BBC Scotland news website: "The disclosure process is meant to be secure. A lot of people are very concerned about giving their personal details out now.
"There seems to be some kind of debate as to what has happened to them. We have been assured that a full and proper investigation is being carried out."
Disclosure Scotland has confirmed that it has not received all of the forms which were sent by Fife Council.
A spokeswoman said: "Disclosure Scotland has looked into this issue on behalf of the council and has informed the council that it has received only three of the applications which were received on three separate dates, some six weeks apart.
"All applications received by Disclosure Scotland are scanned on receipt and entered immediately on to the disclosure system. There are no records of the missing applications on that system.
"Discussions are taking place with the council about establishing an audit trail for all future applications sent to Disclosure Scotland".
The council said the packages were sent by recorded delivery and receipts had been received confirming they were signed for.
Fife Council has said it is looking into the matter and believes that it has taken all the appropriate precautions.
Education service senior manager James McKinstry said: "The documents were sent from Fife Council by recorded delivery on April 11, 2008 and they were signed for by Disclosure Scotland on April 12.
"Disclosure Scotland informed Fife Council that, although they have received and signed for the recorded delivery package, they do not log what and how many applications each package contains.
"Fife Council holds a database detailing the forms that are despatched on a specific date. This information has been supplied to Disclosure Scotland.
"Fife Council is certain that all the forms were sent and is now working with Disclosure Scotland and the Royal Mail to find out where these missing documents are".
North East Fife Lib Dem MSP Iain Smith said: "Questions must be asked as to why both Disclosure Scotland and Fife Council did not realise immediately that each application was not accounted for.
"It is imperative that these missing documents are traced and recovered as a matter of utmost urgency."