Answer:
Because we are saying "Look at our school", we are telling someone to look at our school in real time. For example if I say "Look at my sister. She is wearing a red shirt." I will not say, "She wears a red shirt."
If I am saying "Look at it." then it means that the thing is happening now. So, it will be present continuous. If I say "It stands near the post office" then it may not contain "Look at it". I can say "My school stands near the post office", even when I am telling my cousin where my school is and we are nowhere near it. If I am saying it in real time that "Look at my school. It is standing near the post office", then it means that we are passing by that way and I am telling her where it it, also she can "Look at it". I know I am not able to explain it properly, I don't know how can I explain it, hope you understand it.