Being "expressive" can be difficult if you are just learning to paint. As a beginner, or if you are in an introductory state, the art class probably consists of brush stroke techniques, composition, lighting, color, etc.
When beginning, many painters are afraid to make mistakes, or to "mess up" their paintings.
Being expressive comes with a little risk. Like Bob Ross always said, "Let's get crazy now. Let's add a little rock right there." He always had a general idea about what he was going to paint, but he also just "let it happen".
Oil is a very nice medium to use for expression since it blends well and doesn't dry too quickly. It is my favorite painting medium between the ones I have used in the past (acrylic and water color).
To become a little more expressive, try little subtle things such as stroking the brush more quickly, leaving edges unfinished, making one long line with a brush, and then leaving it, make short little strokes in other places. Change it up a little. Go a little crazy, as Bob Ross would put it. Don't worry about messing up your painting. We learn best from our "mistakes".
I am pretty certain a "mistake" will be tolerated in your class. Take a risk. Add more blobs of white. Use your blank canvas as part of the painting. Do whatever you FEEL like. If you feel frustrated that day, make the picture more violent by scraping away some of the paint. Whatever idea pops into your head, JUST DO IT. Maybe you want the shadow on the pear on the OTHER side of the fruit, on the wrong side of the light source. Just try it! You might like it.
You could also try a helpful expression exercise: pick an object, pick a definitive kind of music (quick, angry, romantic, silly), and paint or draw the object in the way the music makes you feel. The biggest key with this exercise is not to think about it too much. Just let your hand do all the work. Don't be afraid to put a big blob of blue all over that apple!
Another great way to start those expressive juices pumping is to look at other people's paintings and see what they are up to.
Hope this helps. Best regards for your class and paintings!