I simply don't understand how the Georgia school shooting occurr

Stone Harbor - I simply don't understand how the Georgia school shooting occurred... The kid left his class at the beginning and returned with his gun at the end, and then shot up the class next door. How, with all the security cameras and aides and door clearances in an average school, did this transpire with no one noticing it?!? There's a lot of talk about this kids history and his dad being convicted now. But the school is to blame as well. Someone was not doing their job.