an armed "Counselor" is just one sort of armed school employee. Teachers can be another,custodians a third,the principal the fourth.
Having armed teachers (and other school employees)is like having plainclothes police scattered all around the school,at no cost. it's the smart thing to do.
They would be where they are most needed;close to the students that would need protecting,if an attack occurred. All the time school is in session. No need for costly guards.