I followed advice from you guys. I got hold of a AC power supply with voltage selection from 2V to 10V rms, i.e., a transformer. However it didn't work as the transformer is plugged in via mains which means it shares the same earth as the oscilloscope. I realised I needed to plug them in with isolated earth. However I tried all of my main sockets in the room and they are all shared the same earth.
A light bulb moment when I thought of the danger of blowing up measuring equipments if classroom full of novice students wrongly ground the circuits. Therefore, science classrooms must comply to the safety standard by isolating the earth between sockets. So, I took my equipments to the science classroom and connected the wiring including the grounds hoping they are isolated. And then there it was the 100hz full wave rectified curve on the oscilloscope. I was so relieved.
Question now remains is how I could isolate earth in the original classroom? Is there a device I could buy to plug into the mains?