A cellular automaton model based on a time-varying floor field is applied to investigate crowd evacuation from a classroom with two exits. In this model, static obstacles are classified into impassable ones and passable ones. As pedestrians are treated as movable obstacles, the floor field varies with the crowd movement. Each exit produces its floor field, and a pedestrian selects his/her target exit by comparing values of both fields in his/her position. Numerical simulations are performed to study the evacuation process in consideration of exit width and desk layouts. It is shown that the key factor of exit choice is the distance to each exit in the evacuation process. Initially students prefer to enter the nearest aisle. In an evacuation process, few students in the middle of the classroom turn to another exit due to the congestion near the previously selected exit. Once they are approaching their target exits, they usually do not change their choices again. It coincides with the daily experience. This model can be used to simulate crowd evacuation from a multi-exit room with complex inner structures.