In many cases, native English speakers can often determine whether some sentences are written correctly based on how it sounds. Whenever they read out a sentence, they can determine if the sentence sounds correct or wrong. Often times, people do not want to split an infinitive.
An infinitive is a verb phrase that begins with “to.” For example, in the sentence “I went to study at the library,” the infinitive is “to study.” Splitting an infinitive is when another word is inserted in between the word “to” and the verb.
In this case, you would not want to write the sentence as “I went to not study at the library.” Therefore, a correct sentence would be “In school, student have to study hard in order not to fail their classes.”