The RHS has a feature on gardening for wildlife which, although about homes, applies to schools and other workplaces as well.   It says:

It isn’t difficult to encourage wildlife without compromising the way your garden looks.  Small changes to your garden can bring major benefits for the creatures that call it home. Some will actually save you time and resources, and all should bring in more wildlife to watch.  Remember: gardening for wildlife doesn’t mean leaving an untidy mess of only native plants.