In this sentence, connoisseur is being used as a noun. "Jerry was a [...]" can not be followed by an adjective when used this way. Jerry was a red, Jerry was a stupid, Jerry was a grumpy. These are all followed by different adjectives and do not make grammatical sense. Connoisseur may also not be a verb because these following would not make much sense either: Jerry was a running, Jerry was a took, Jerry was a walking, Jerry was a jumped.
The actual meaning of a connoisseur is a person who has good sense or judgement. A critic when it comes to tastes in things like art, etc. Without knowing this definition, however, the above were ways you could figure something like this out next time.
Sometimes in English, a verb is used as a noun. When the verb form is altered and it serves the same function as a noun in the sentence, it is called a gerund.
The plot is about how middle schoolers evolve. The setting of the school shows the everyday life of the main middle schooler going through the struggles of being a preteen and the preparation for high school. (If this doesn’t help google the plot of the story and think about the setting and the the relation.)
