A lot of schools have exterior walls that are concrete filled in the US, especially true in tornado and hurricane areas. Interior, I think they went to concrete filled for most of them but you would have to check building codes when the structure was made.

A hollow core cinder block wall should be adequate protection from a grenade.

That is lethal range, range of potential injury is 50m

Depends on the wall. Solid concrete, safe. Residential interior wall made of plaster, not safe. Brick wall, safe. Near a window, not 100% safe. Do you know the composition of a wall? Chances are no, so remove ass from the area.

They have an injury radius of about 50m, so 100m of free air is safe.

If the picture is the actual thing, it doesn’t have the detonator and fuse. You could throw it in a fire and not have a problem. You could hit it with a hammer and not have a problem. If you threw it in a fire and hit it with a hammer, you would ruin your whole day.

Most likely it has been fully demilled and is just a grenade shaped object that only is a threat if it is thrown at you or dropped on your naked toes.

If you aren’t certain an explosive device is inert, don’t pick it up and clear out anybody while you wait for an adult with gallows humor to clear it. How far you need to un-ass from the area depends on the size. A grenade? 100m. Anything else, the harder it would be to fit it in your ass, the further you need to GTFO.