My instructor was an hour builder (he's got a REAL job now) but he still taught me to take off and land from grass (so they all aren't that bad). My home airport has two short grass rwys and my club doesn't have any restrictions on flying from grass.
But I totally agree with kolibear that there are a lot more variables to deal with.
It's all down to airmanship at the end of the day. Schools don't necesarily have to teach people how to land on grass. What they have do is ensure that students have a strong sense of airmanship before they are let loose.
Unfortunataley many schools offer substandard training and then make up ridiculous rules and myths to cover their a$$es
Capt. m