Send Clowns...two very interesting theories...I have never heard about fuel imbalance being an issue until your post. Particularly as the spin in question occurred at the tail end of a 'Precautionary Search & Landing' lesson....so many years later there's no way to remember the fuel load but it would seem highly likely that it was reasonably balanced as the C150/152 series fuel systems have a 'both' selection...I'm 6'2" and the student was not short...both being VERY young at the time we were also as skinny as rakes...unlike now in my case at least
I never got a definative answer on why that aircraft failed to respond...one theory was it had been 'built up' from two aircraft wrecked in a cyclone in Queensland' and may not have been rigged true. More likely to have been a combination of small things that all summed up on the day.
Actually as it was a fairly old C150 can anybody actually confirm that they, like the later model 152s actually did have a both setting for the fuel tanks...I'm trusting to nearly 20 year old memory.