ChiSau,
For the definitive answer, check the Pilots Operating Handbook (POH) for the particular aircraft. The school should be able to tell you where to find it.
I've never flown a C152, so I can't comment on that type. But, on the PA28, I don't think it's to do with the margin above the stall, as Aussie Andy says, although he's close. What happens is that, with less flap, the stall speed is higher. Therefore the approach speed is higher. Therefore, in gusty conditions, the gust speed is a lower proportion of your approach speed, and so gusts will have less effect on the aircraft.
Personally, I use full flaps in a PA28, except in gusty conditions. This gives a lower stalling speed, hence a lower landing speed, and it takes less time/distance to brake on the runway. Now I'm flying a Europa, and I have no choice but to use full flaps, since the gear is connected to the flaps - if the flaps are up, the gear's up too!
FFF
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