This is a great looking cannon. Fires a 40 pound projectile at 984.25 feet/sec. Probably not good to be anywhere near the interior wall though. Like within 15 feet.
Might be best to have one in every class room to use to get out with. May be slightly costly to repair the building after use though.
I would be a bit concerned if talking to a "defense consultant" that really has no clue as to the physics of projectiles.
You could blow a hole without a lot of structural damage or need more than a 4 foot "safety" zone inside the wall and not have any major rebuilding of the wall after you were finished using the hole.
People on the outside would be pretty safe within a very few feet of the impact though so it would be good for storming a room/building.