I don't see an entirely happy ending in Barthelme's short story. There are two fundamental issues at play in this ending. The first would be that the kids really do not receive anything in way of answer about the issues of death and life that confront them. They have experienced death in so many forms. "The tragedy" as well as the death of classroom pets and plants all confirm an underlying reality that this particular group of children have not experienced the affirmation of life. They try to open this existential discussion with their teacher. This is to no avail as he cannot provide much in way of answers, or even guiding questions. He is incapable of providing an emotional atmosphere in which the children can understand the complexities of life in the face of death. For his part, the teacher is not able to really ascertain how to guide them and how to address these questions himself. The embrace of the teaching assistant in the end of the short story does not really answer much of anything. It seems more forced and something that is not constructive as much as a way to silence any further discussion.
The teacher's fears and the students' fears converge. The arrival of the new gerbil to cheers almost perpetuates the silencing of voice that is a part of the ongoing dynamic in the classroom. It is here in which I don't see a happy ending. There is not much in way of understanding gained. Rather, there is a continuation of the same pattern that fails to understand how to live life, to look at it for what it is, in the face of death that exists and is around individuals. The presence of a "spiritually sullied world" is seen in the ending, where the cheering of the gerbil indicates that since little has been learned and understood, the process of embracing life only to see it flicker away will continue.