This is not a big deal. When I was in HS they had a similar thing going on....there was no panic as we are never panicked and everyone knew basically what was going on after a few minutes. They did a drug search for about 3 hours.
Again...not a big deal at all. They have practice lockdowns all the time and the schools are better off for them. The worst thing that could happen is an actual emergency takes place and everyone is hysteric and running all over the place.
Of course, it is important not to go over the top. Here in my district we have codes with a certain inference to them. They pretty much call out the level code and tell the students to stick with procedure, lock the doors and remain inside. No one panics, etc.
Now if they had gone on intercom and said "Hey there's an intruder! Hide!" Then yeah...people might get nervous.