Hi,
I don't know what the standards say about this procedure but I can tell you that it is a tried and tested technique. I have come across it in a number of premises with similar problems to your building. There's nothing wrong with it if it is done correctly. Staff should be plentiful and they should have no difficulty in immediately finding their key. Visitors should either be accompanied or reasonably closely attended to by the staff so that they shouldn't have difficulty immediately contacting a staff member to raise the alarm. Contractors need special consideration depending upon their locations and upon the proximity of staff to those locations. If they are working on their own in the building, especially out of term time, they should receive some induction training on arrival at the building anyway - the issue of a key (with a giant fob, say, to stop it being misplaced) could be part of this induction.
Believe me, it can work. However, your building seems to have adopted the policy in a half hearted way. This is not satisfactory and must be remedied one way or the other. They really have no choice but to fully implement the correct procedures (probably including some staff training) or to abandon the key operated call points and adopt some other technique to control malicious use.
Stu