Using 'look on as' indicates a personal opinion or belief, not a general fact. The teacher looked on as a mentor by some students, but not all.
Pro Tip 2/3
Context Matters
The context can change the meaning from positive to negative, so consider the situation carefully. In the debate, he was looked on as a skilled opponent.
Pro Tip 3/3
Cultural Nuances
Cultural background can influence how someone is 'looked on as' in different societies. In her hometown, she was looked on as a pioneer.