Mr Muller said after the farewells and parades with plenty of "go get him mate", the adventurers arrive overseas keen to get out on the field, and "nail this thing", hoping to be home by Christmas.
He asked the students, "When do you think the fear would start?"
At the first briefing ... or hearing the first explosions ... or the first bullet rips past your head? ... Or when you saw the first guy get hit? Then the realisation, 'oh my God, what have I done?'
"Then the hiding, the endless digging, then the waiting knowing you have to go back over the top, and face it - death."
Mr Muller urged the students to take time to pause and reflect.
"All gave some, some gave all, lest we forget."
Student Jackson Currie, 16, said he enjoyed Mr Muller's speech and coming from an school old boy made it extra special.
"It definitely put into perspective for me what huge sacrifices the Anzacs made for us and that we must never ever forget that."
The assembly concluded with head prefect Te Puruhi Moana and house captain Hamish Hahunga reading the ode, a bugler playing the Last Post and Reveille.
Mr Hahunga will be giving a speech at today's Anzac commemoration service at Tauranga RSA.