The only reason I can think of for schools etc banning their aircraft from grass is the uncertainty of the factoring effect on take-off and landing roll.
With a hard runway you know what you have got and the only effects you need to consider are slope and wind, which are obviously common to grass strips too.
With a grass runway, it starts to get a bit subjective, is the grass long or short? Is a bit of dew classed as wet grass or dry? Theres been no rain for a few days and the grass is dry, but the ground is still soft, does that class as dry or wet? Its all a matter of opinion.
Now you can insist that your students or renters either factor in long wet grass x1.3 (safety factor) whenever they use a grass strip and call the result their T/O & landing rolls or just ban your aircraft from grass all together.