u Gardens are a huge bore for the uninitiated: think of interesting angles, like the environment, organic food, oxygen and nitrogen, colour and texture, planning, design and aesthetics.
Grow plants or flowers in school before you go: even something as simple as cress seeds, or 'breed' different types of flower, so that pupils start thinking about the plant environment, genetics, climate, how humans fit in.
Divide the garden, especially if itis a biome, into different sections and get individuals and groups to read up about 'their' plants, before setting off, so that, between them, they know as much as their guide; then compile a class project book from pictures, written accounts and photos.
Don't let anyone fiddle with the plants: children should learn to respect what they see and the efforts of those who work there.
Check obvious allergy problems: hay fever and asthma, especially in the pollen season, can cause difficulties, but don't get too neurotic about them; just do what you can to avoid discomfort for those who might suffer from an allergy.