Institution of Martial Law?

My high school recently instituted plans to give every student a key card starting next year which will let them into the school, which I presume would be locked to everybody else besides a few main doors. This raises a few little red flags in my head. How do I know that this same system could not be used to lock us all into the school? Or lock us out? The system would basically be locks on the doors with key card entry on the outside. Why could they not just lock it from both sides? Our current system seems to be working fine, and while I understand that somebody could just walk in off of the street at the moment, there are plans for such an occurrence, and they seem to be fine. Also, the school will not let us carry cell phones on our person (although most of us do anyway). We are not even allowed to have them in our backpacks. They say either keep them in lockers, or don't bring them. Recent advances towards enforcing these rules have made me somewhat concerned. Call it paranoia, but how much easier could they make it to lock us all in with no means of outside communication?