The appropriateness of a teacher using a student bathroom can depend on several factors, including the school's policies, the context of the situation, and the specific circumstances surrounding it. In many schools, teachers and staff are expected to use designated facilities for adults. Using a student bathroom could be seen as unprofessional or inappropriate, especially if it creates discomfort among students or suggests a lack of boundaries.
However, there may be situations where it could be acceptable, such as if there are no other available restrooms or for specific reasons related to health or accessibility. Ultimately, it would be best for school administrators to clarify and communicate the appropriate policies to ensure a professional environment for both teachers and students. If there are concerns about this behavior, it might be worth discussing them with a school administrator.