As mains current is AC, you'd have probably been producing a mixture of Oxygen and Hydrogen at each electrode in just the right proportions to recombine back to water (or rather steam) very violently. Assuming that you were in the UK, the voltage would have been 230v (not the piddling 115v they use over the ditch) so I think that you were lucky that your mum did turn up - and that you kept those electrodes far enough apart to prevent the resistance becoming too low. I expect your teacher used about 6v DC, although it's usual to add something to the water to lower it's resistance.