The schools opened their doors to council auditors and showed them around during a series of on-the-spot inspections this year.
However, they failed to check that the people they were showing round were who they said they were.
Surprised inspectors gave security at city schools a largely clean bill of health late last month - saying alarms and procedures for reacting to those alarms were in place at all.
But in a report revealed today they added: "The auditors were shown around several schools without being asked for ID.
"Whilst visitors were usually escorted during visits, in one instance the auditors were able to walk through to the assembly hall unchallenged while it was in use by children."
Bosses also highlighted that staff and pupils sometimes left doors and gates open, undermining security.
At several schools they found staff were not wearing ID, "making it more difficult to recognise visitors and potential intruders".
Scotland's schools aren't quite fortresses but they have all beefed up security since the horrific events of Dunblane in 1996.
The auditors have recommended that all schools in Glasgow now put in place procedures for checking the IDs of all visitors.
Senior staff at the school where auditors got in to the assembly hall unchallenged have been told to review security.
A council spokeswoman said: "We welcome the broad findings of the report.
"The safety of pupils and staff is paramount."