surveillance cameras are a joke. first of all, they don't prevent anytning at best they record events. Secondly, for them to be any kind of warnign device, they have to be monitored on a constant basis by people on perpatual alert. difficult to do for long periods fo time without going stir crazy.
And going into lockdown mode takes time, in a situations where time is critical, how long does it take to locate and contact someone with lockdown authority - and how long does it take that person (who will most likely have questions before giving the order) to order a lockdown?
Then there's the mechanism for the lockdown. how do you
communicate a lockdown? Via public address? that also alerts the perp. Then you have to physically do the locking (Many doors are not kept locked, and many teachers don't have keys anyway.
And what do you do with people in public areas? In the hallways? rest rooms? Outside?
And do you really want to notify the police everytime someone "strange" (whatever that means) approaches? Maybe it's a parent you just don't recognize? or the new sales rep for one of your vendors? Or any number of other scenarios?
All you'd do would be to desensitize folks to alarms - for false alarms would be rife.
and you never explained why you wouldn't want your teachers armed. what are you afraid they'd do?
I had a history teacher in H.S. that I just knew was going to show up armed one day and whack half of the class.