(I'm basing this on my experience in government, where these issues came up often. For example, when I worked in the Federal Aviation Administration's Office of International Aviation, we often dealt with safety and security issues of aircraft, airports, personnel, etc.)
When a distinction is made, "security" refers primarily to intentional actions (e.g., a cybercriminal trying to hack into an air traffic control system) while "safety" more often refers to unintentional events (e.g., a bird being sucked into an aircraft's engine). Note that this distinction isn't very strict and there is a lot of overlap between the terms.
In general usage, "safety" can refer to any threat to people while "security" is very rarely applied to unintentional actions. For example, a terrorist attacking a plane could be considered a threat to either the "safety" or "security" of the passengers while a lightning strike would normally be considered a "safety" issue but not a "security" issue.
In the context of a classroom, I'd imagine that "safe" meant that there was no unintentional threat (the smoke detector was working, there was no asbestos in the walls, etc.) whereas "unsecure" meant that there was a possibility of an intentional threat (the doors didn't lock, there was no security guard, etc.). Without more details, though, the meaning isn't entirely clear.