If you mean for a school garden, then I think the first thing to do is decide on a theme .

Possibly a wildlife garden for the birds and bees and insects.

Or a garden with edible plants ; cordon trained fruit, , soft fruit, herbs, vegetables, edible flowers etc.

Or a historical garden eg a Tudor garden and research the history of gardening at that time.

Even, a tall garden. Who can grow the tallest sunflower, delphinium, hollyhock, etc.

Depends on the space you have and the kind of students you have too.

Some garden centers and nurseries will even donate seeds for a school garden--possibly in hopes of training up future customers!