Motivational Interviewing (MI) is appropriate for all grade levels.
MI could help address low motivation/engagement/work completion, but it would also be helpful when addressing disruptive, disrespectful, or noncompliant behavior; peer difficulties; or attendance concerns.
Because MI could be used to address such a variety of concerns, indicators of success will vary.
For example, if you use MI to address low work completion, you would expect to see the student's rate of work completion increase.
If you use MI to discuss peer difficulties the student is having, you would expect to observe fewer contentious interactions between the student and their peers.