Infinitive is the verb in its normal and neutral state, it is always presented with the auxiliary "to" which is positioned in front of the verb. In this case, we can see that the infinitive is overbo "to make", the infinitive phrase being "to make good grades in school" as shown in the question above. This sentence takes the position of an adjective, as it characterizes the "responsibility" presented in the sentence.
An adjective is a word that characterizes another word. Therefore, the infinitive phrase is characterizing what is the responsibility of an individual.
