Final answer:
The issue of greatest concern is the potential for a dual relationship which can lead to a conflict of interest, impair professional judgment, and violate professional boundaries in psychology.
Step-by-step explanation:
Based on the information provided, the ethical issue of most concern is the potential for a dual relationship. Dual relationships occur when a professional, such as a psychologist, has a relationship with a person in more than one context. For Jason, a psychologist attending a parent-teacher conference, considering the teacher's request for marital therapy would represent a conflict of interest and could impair his professional judgment. The American Psychological Association's (APA) Ethics Code cautions against such relationships that could lead to harm or exploitation.
In this scenario, it's important for Jason to maintain professional boundaries to avoid the complications that dual relationships can bring. Instead, he should refer the teacher to another qualified marital therapist. There are also concerns about confidentiality and the ability to remain impartial which could be compromised by entering into this type of professional relationship with someone he already knows in a different context.