Based on the novel The Lekki Headmaster, what grammatical rule was the source of disagreement between the MD and the principal?

The grammatical rule at the source of the disagreement between the MD and the principal (Bepo) was subject-verb agreement with phrases like "as well as." The dispute arose from the sentence "Ade as well as Jide comes early." The MD sided with the parent, Mr. Guta, believing the verb should be "come" (plural) to agree with both "Ade" and "Jide." The principal, Bepo, defended Mr. Fafore by explaining that the grammar was in fact correct.

The rule is that "as well as" is not a coordinating conjunction like "and." It does not create a compound subject. Instead, the verb agrees with the subject that precedes "as well as." In the sentence "Ade as well as Jide comes early," the main subject is "Ade" (singular), so the singular verb "comes" is correct. If the sentence used "and" instead ("Ade and Jide come early"), then the plural verb "come" would be required.

This is a commonly misunderstood grammar point, and the novel uses it to illustrate how confident but incorrect assumptions about language can have serious consequences, including the wrongful dismissal of a competent teacher.