A trusted mechanical solution, 9K Series with Intruder Function by BEST, is used to equip a classroom door that eliminates a major point of classroom vulnerability. BEST’s 9K Series enables teachers to discretely lock the exterior lever or thumb piece trim from inside the classroom should they hear or be warned of a disturbance outside the classroom. In doing so, the teacher draws little attention to the students in the room while simultaneously increasing the level of security. Essentially with the use of the 9K Intruder Lock, teachers and staff will know that their door is locked and secure via the visual indicator shown on the 9K Intruder Lock, while the interior lever or exit device can simultaneously remain unlocked, providing safe egress, if needed. The function requires minimal installation time, limited disruption to existing door hardware and is a highly cost-effective product to better secure classrooms during the threat of an intruder and emergency situations. Thousands of education buildings in the United States are already equipped with BEST 9K Intruder Locks.