Go out into the field looking for textures.
It would be great if you could provide a camera for each student or group. If they cannot have access to a camera drawing textures is an interesting way to challenge students. Spend a lesson collecting drawings of texture and then in the next lesson you can ask them to choose one to paint.
They could use a variety of techniques so have sponges, scrapers, straws, atomisers and challenge them to come up with other interesting implements with which to make marks and impressions.
It is very challenging for a student to try to convey the impression of a texture with out giving any other visual clue to where the texture comes from.
For example can you draw
Ask the students to come up with challenges and perhaps play a Pictionary type game calling out textures instead of words and phrases
The tricks when trying to achieve texture is to focus on
These types of challenges are fun. When you have practiced, experimented and explored it is time to set a real drawing project. Ask the students to choose a still life sporting high textural contrast. A cob of corn surrounded by cut onions is a good one. Or a horse with a silk cloth under its saddle. Angels with white wings is taking the idea to a more difficult level.
What ever the project don’t forget to draw the student’s attention