Your students aren't writing opinion pieces what you want because you haven't given them a student friendly visual rubric. This visual writing rubric will support your students in their English Dual language classroom by giving them an example of what their opinion and argument should look like for both writing and illustrations. Set the expectation and a goal for each student, and give them something to work towards.
 
This is included in your Pa'Lante Membership 
 
Highlights:
Print in your classroom and have students self and peer evaluate their writing work
Set expectations for writing
Display all year long
Provide goals depending on the the student and activity.
I number them from 0-7 and students write the number of their work on their paper.
Makes great anchor charts
Includes common writing errors of 1st and 2nd graders
BONUS- Includes some writing papers!