The school my daughter went to last year has its outer gates locked at 15 minutes past the start time, access to the school is then by a remote controlled metal gate with a security camera pointed towards it. The gate is situated next to a brick wall so there's no way anyone can shoot through the reception entrance either.
Once past that gate there is an intercom system for the reception as well as the reception door having a remote release and it's also locked at 15 minutes past the start of school. There is a 6ft metal fence, or a wall at some places, running all the way round the school as well. It wouldn't stop someone who was hell bent on destruction, but it would certainly hinder most people for long enough to make an emergency call.