Final answer:
The statement is an example of the fallacy of affirming the consequent, where the teacher assumes that if the test grades improved, then it must be true.
Step-by-step explanation:
The statement in question is an example of a logical fallacy known as the fallacy of affirming the consequent. This fallacy occurs when someone assumes that if the consequent of a conditional statement is true, then the antecedent must also be true. In this case, the teacher is assuming that if the test grades improved throughout the year, then it must be true. However, this is a flawed assumption as there could be other factors contributing to the improvement in grades.