Many places, where I work included, preach and teach some variation of run-hide-fight. That is, in the event of an active shooter incident, evacuate the area if you can safely. Hide/barricade if you cannot, and as a last resort prepare to fight for your life. (*)
In a situation like this, where you are successfully barricaded, be smart. Wait. Know the statistics and procedures for your area. If the average response time is 10 minutes, know that you probably won't see an organized SWAT or large scale response to an active shooter for twice that. Be prepared to wait 20 or 30 min. If someone comes knocking at 5 min, claiming to be a LEO, first just keep quiet. Don't give away that there are any targets on the other side of the door. If some fool speaks up, tell the alleged LEO that you are all fine, no injuries, and you are going to wait for x minutes before opening the door.
(*) My plan, realized/developed years ago is a little different. Due to security measures, it is virtually impossible for anyone to get to where I work who does not belong there. The only feasible scenario is a disgruntled employee. That means he/she would've had the same exact training I've had and be expecting the run/hide/fight trio. Never be predictable, never give up an advantage. If something goes down, I'm not running, I'm not hiding. I'm hunting. I'm old enough I've done everything I need to do. If it doesn't turn out, so be it. But if I can surprise someone and save lives, great.
Another school in Michigan does intense drills/training with teachers for such situations. Some churches in the same area train for active shooters, too. IMO, not drilled enough.
In the 1991 murders at Luby’s in Killeen, Texas one of the kitchen staff hid in the dishwasher for hours until found by deputies. Smart guy.