On my first flight into C airspace with my instructor (as a private pilot
student) I didn't realize the policy of the tower was to ask for GA planes
to shut off their transponders while doing circuits.

Tower:
"cessna 1234, squawk standby"

either a lame response from me, or no response

tower repeated the call at least 2 more times before my instructor asked if
I was going to turn the transponder switch to standby as I had acknowledged
the call and he knew I heard it.

I thought I was standing by for a new code and they were just taking their
sweet time making one up.