School have tried putting cameras at every door and putting buzzers at the doors and this has worked for the most part, but people still find a way around it. Some ways that could improve school security would be to install a fence all the way around the school and put barbed wire on top of the fence to keep people from jumping the fence. Hiring security guards to patrol the school and outside the school would help keep people from trying to break in to the school and would help supervise the students. Allowing teachers to carry some type of weapon would also help in case someone does break into the classroom, this would allow the teachers to stand a chance against the intruder and fight him off. The easiest way would be to simply lock the classrooms. This would not allow a person to simply walk in to the classroom and start firing on the class.
If nothing is done to improve and strengthen school security, then nothing is going to stop people from repeating what happened in Connecticut. Also if no improvements are done to school security, more parents will think about homeschooling their children and the population in schools will decrease.