I appreciate the question, but you need to realize that from an ethical point of view, I cannot make a specific treatment recommendations about an individual for whom (1) there is no consent for behavioral services (2)I am not the behavior analyst authorized to provide services, (3) I have not done a full record review and assessment, and (4) I have no information on the possible function of this individual's behavior. Behavior analysis is a very individualized form of treatment, and the function of any problem behavior must be assessed before an intervention plan can be suggested. People do things (or refuse to do things) for a reason. That reason can be assessed, and their history is very important in that assessment.
That being said, usually (not always!) a person does not engage in assigned/requested tasks for one of two basic reasons: either they get a ton of social attention (which they greatly value) for refusing, or something about the task is aversive, i.e., they find the task unpleasant or too demanding, or their history of reinforcement for engagement not very strong. What seems easy to you ar me might be very difficult for someone else, or they have a history of unpleasant things happening when they have engaged in the past. Perhaps the task requirement is too complex, or too much effort is being asked -- and so now task demands are refused because they have very little history of getting good things (reinforcement) for engagement.
As you might suspect, if the reason for not engaging is to get attention, the intervention would be very different from a situation where the failure to engage is due to the individual seeking to escape or avoid the task requirements.
I would suggest you look at "The power of positive parenting" by Glenn Latham to get some ideas on how to encourage children and adolescents to engage in tasks in a very calm, non-threatening and effective way. We often start by (1) making the requirement for success very clear, and (2) making it initially very easy for the individual to succeed in doing just a little of the task (depending on the functional level of the individual, sometimes even just "look at me" followed by eye contact is enough for earning reinforcement) -- followed by positive social interaction and possibly access to some preferred activity and/or tangible item. Then we fade-in the demand requirement (either by increased amount or increased difficulty) as the individual learns that completing tasks is (1) reasonably easy and (2) followed by some environmental change that is valuable to THEM (i.e., reinforcement). Another way to increase engagement is to offer the individual a choice of activities, where they get to choose what to do, in a preferred order.
But again, What I just wrote is not a suggested treatment plan! It is only one way to proceed with a particular client who is refusing to engage due to their own personal history. I cannot stress how much a formal individual assessment is absolutely necessary to discover what is going on here, and why the individual is refusing. Without that, we are just flying blind, and proceeding with a treatment plan would not be an ethical course of action in any professional discipline. Think of a doctor performing major surgery on someone who complains of stomach pain -- but without the benefit of any blood tests, x-rays, MRIs or other appropriate medical tests.
I totally agree with Dr. Freeman. I would also add that the starting of any task is essentially 90% of the work for many of the students who have emotional and behavioral disorders, any type of procrastination issues, or emotional dysregulation. What could make this easier for both of you is to define the initial part as a step of the assignment due before the assignment. This may be seen as less threatening and practice which is done in class. Practice on the first few sentences allows the start of an assignment with more support.
With the assignment already started and the first sentence inclusive of the main idea, having that increased supportive step in class thoroughly demonstrated the ability to brainstorm. Your assistance or a buddy system during the first class could also continue with the first paragraph if this was less difficult than expected. By decreasing the student's ability to rely on habitual patterns of procrastination, the starting of an assignment can increase the likelihood of assignments being completed.
I would further recommend that students with learning support or emotional support email both you and the learning support or emotional support teacher following the class to ensure that any first sentence started is not "lost" and can be worked on at a later date.