In order to be art, a piece doesn't have to have meaning. A portrait of a person can simply be a portrait of a person. However, viewers may find meaning in a piece that the artist didn't intend. Art is subjective. People may find different meanings depending on their life and experiences. In regards to trying to encourage students to make art with meaning, lessons with open themes definitely helps. if you tell a student to draw a tree you aren't going to get any meaning behind it, unless someone really loves trees.
This dum dum drawing has literally no meaning, but that doesn't mean its not art.