Plants, or members of the kingdom Plantae, share several characteristics. Most importantly, from the point of view of life on Earth, they are photosynthetic autotrophs: they make their own food utilizing energy from the sun, providing energy in the form of food for heterotrophic organisms. In the process they release oxygen, maintaining the atmosphere needed for oxygen-dependent organisms to survive.
Plants are multicellular eukaryotes. Their cells contain a nucleus and membrane-bound organelles, including the chloroplasts, where photosynthesis takes place.
Plant cells have cell walls made of cellulose, a carbohydrate.
Plants are not motile. That is, they are unable to move around on their own.
Plants reproduce sexually. Their reproduction is complex and involves alternation of generations.
As you see, plants have more than three characteristics in common, and there are other common characteristics I did not include. I hope you will be able to identify those most relevant for your class. More information is available at the attached link.