"My teacher had mentioned that resonance would occur here due to the presence of a positive charge if benzene is attached. However, if we consider two bonds without breaking them, there will be one pi bond. So, will there be no resonance due to it?"
Though I am a layman on quantum chemistry, benzene (C6H6) is negatively charged inside the ring, and positively charged on the outside. The Pi electrons in NO2 tends to be attracted to benzen and have some tendency to break. The resonance would be incorborated into total bond strength.
"My teacher had mentioned that resonance would occur here due to the presence of a positive charge if benzene is attached. However, if we consider two bonds without breaking them, there will be one pi bond. So, will there be no resonance due to it?"
Your question is not clear. Could you rephrase it?
