i guess I didn't read the original question like others did and I didn't read all of the answers. So.....
Inlwhitt said she is using the walking foot. So therefore stitch length has nothing to do with pushing the work thru or speed of the motor.....So lets start over. However, there is a sentence after walking foot that puzzels me. Are you doing both walking foot and some free motion. This is confusing.
If only walking foot - I suggest you lengthen the stitch slightly to accommodate the extra layers of fabric. My machine is set on 2.5 so I lengthen it to about a 3. Fee dogs up and hold the fabric so there is no drag.
Should give a consistent stitch.
If you are using the walking foot - you are not responsible for the length of the stitch. The feed dogs do that.
Also you said "this week we are supposed to do th quilting.......etc" Should you be doing this in class or was it homework. If homework then I would assume the instructor gave some information about how to do it.
If not homework
then perhaps should wait for the class for instruction.