This is offered for your consideration during revisions. Please take no offense from the volume or directness of my comments.I wonder if opening a story in a summary flashback is the ideal way to start this story. It's working okay, although I'm not comfortable with a story that opens in the pluperfect tense. Too many stories begun in that tense end up with an annoying surplus of the word had,. If I were skimming this story, after the third had in the first thirteen lines, I'd put it aside.
Nevertheless, judicious use of pluperfect tense works well in summary flashback, although opening a story in a pluperfect tense is a challenging one to maintain and might lead to ever more awkwardly complex plupluperfect tenses later on. (An example of a plupluperfect construct, Had to have had a premonition, yikes!) Consider recasting into present past. The other time contexts of the existing pluperfect phrases provide effective time management so it wouldn't change the meaning. For example. [The >Friday< Jonathan Tempos died trudged along] [Jon reluctantly grew accustomed to in his >five years< of teaching] [>after< classes were dismissed]
[mundane] has two diametrically opposite meanings; the earthly realm as opposed to the metaphysical one, or dull. Consider a more precise adjective, like droll, boring, tedious, etc.
[every other weekday] every other day is an idiom for skipping a day between days. I read the phrase to mean Monday, Wednesday, and Friday. Consider recasting to something like 'every workday.'
[He loathed the job, but there were bills to be paid when you developed a family and a gambling habit in community college] that sentence is heavily freighted with context, loathed job, the onus of bill paying, a burdensome family, a demanding gambling habit, a community college education. I suggest recasting into separate sentences.
[there were bills to be paid] passive and infinitive future perfect verb construction, unnecessary tense shift. 'there were bills to pay'
[when you developed] is a marked change in tenor and a change in person from third to second. I suggest simplifying or recasting the sentence. Developing a gambling habit is a common idiom, but developing a family is awkward and emotionless. The family and the gambling habit are burdens. Community college is not necessarily a burden; however, an associate's degree doesn't typically qualify a person to teach.
[At 4:15, fifteen minutes after classes had been dismissed - and exactly 8 hours] three exactly precise time marks are redundant, consider simplifying. I'd at least recommend omitting the [and] and replacing the dashed interruption with a comma. Exactly is a superfluous adverb.
The prefered punctuation style in fiction writing for a dashed interruption is two hypens with no spaces. 'word--word' However, the narrative didn't change direction or interrupt a thought or dialogue. The phrase set off by the dashes is an appositive clause. A dash is not indicated in these instances. Commas are the prefered punctuation for setting off appositive clauses. 'At 4:15, fifteen minutes after classes had been dismissed, exactly 8 hours'
[Late '80s Marquis] sentence case 'late' unless part of proper noun. The context confused me. I didn't know whether it was a late model Marquis or later year in the '80s Marquis. ['80s] is a time reference for the story's milieu. Late model means recent. If the Marquis is 30-years-old, I'd want to know when it's first mentioned.
[would barrel] unnecessay tense change to future perfect. The word [before] already makes the time transition smoothly. Consider 'barreled'.
[sat at his desk tapping a red pen thoughtfully against his temple] consider whether sat at his desk is necessary. 'Jon tapped a red pen against his temple' eliminates the gerund verb and conveys static but dramatic meaning. [sat] is passive and non-dramatic action.
[thoughtfully] is a superfluous adverb. If the word is needed, the sentence flow would be smoother if it wasn't separated from its verb. 'Jon thoughtfully tapped the red pen against his temple.' Tapping a writing implement against a temple is a simple but complete scene-building line showing contemplation. [thoughtfully] tells the reader what the action means, as such it's exposition.
[as he skimmed a student report] [as] is not typically a proper conjunction. 'while' is prefered for simultaneous actions. For that matter, I suggest beginning a new sentence to do away with the awkward conjunction altogether. Tapping his temple and skimming the report appear to occur simultaneously when they're joined in a sentence; however, separating them would show them as the separate actions I feel they are. He's making a gradual shift in focus. He's going from absentminded thinking to passive involvement.
[student report] the context indicates a possessive case 'student's report'
[His gaze kept crawling up the page and over the rim] gerund construct that implies repeated action. I think it would be more dramatic and robust if it happened once and would then lead directly to Jon's single fixation on the clock. 'gaze crawled'
[rim of his glasses] number agreement between objects, 'rims'
[class room] compound word 'classroom'
[The seconds passed in jerky muted ticks] is a beautiful line, but a little metaphorically awkward. The clock's second hand is what jerkily, mutely ticks. And a comma separates noncoordinate adjectives. 'The second hand ticked in jerky, muted steps.' or some such.
[door. . Finally, He let the paper drop from his hands and flutter down to join it's comrades that littered his desk]
Extra period and space after [door.]
[Finally,] when this word begin a sentence it's a sentence adverb, a dangling modifier, or a discourse marker in dialogue, examples of common discourse markers are; like, well, now, okay, etc. Like with most sentence adverbs, dangling modifiers, and discourse markers, I think finally is superfluous. And finally makes me think the writer is telling me the story is about to begin. Consider omitting.
[He let the paper drop from his hands and flutter...] consider simplifying. The only action Jon did is drop the paper. He didn't let it flutter or join its comrades. The paper did that on its own. [his desk] his is an unnecesary pronoun, especially among two others in the same sentence. 'He dropped the paper. It fluttered onto the pile of reports that littered the desk.'