John Sul­li­van had 2020 cir­cled on his cal­en­dar for a while. For all the right reasons.

“(We) prob­a­bly had our best senior class that ever came through the school,” the Vista Peak Prep foot­ball coach said Mon­day after the dead­line passed for Col­orado high schools to declare whether they would play foot­ball in the fall or the spring. “This was the year we real­ly had cir­cled to do well.”

Despite requests from Sul­li­van to his supe­ri­ors to start prac­tice this week, once the option became avail­able, the Bison will be flex­ing their mus­cles in the spring. Vista Peak opt­ed to play dur­ing CHSAA’s Sea­son C, which runs March to May.

“My stom­ach has been twist­ed for about two weeks straight,” Sul­li­van said. “And it’s still twist­ed. I can only imag­ine how the kids feel … Although we’re dis­ap­point­ed in the deci­sion, we respect it and we’ll pre­pare for the spring.”

At least they’re not alone. Mul­ti­ple sources told The Post on Mon­day that a hand­ful of Front Range schools in Class 5A, includ­ing Moun­tain Range, North­glenn, West­min­ster and Boul­der High are still slat­ed to play foot­ball in the spring. Den­ver South, Gate­way, Auro­ra Cen­tral and Fal­con are report­ed­ly among those in 4A who’ll join the Bison in Sea­son C.

Den­ver Pub­lic Schools and Auro­ra Pub­lic Schools last week declared that they’d play foot­ball in the C peri­od. But it’s a small group, statewide, with a vast major­i­ty of schools elect­ing to play in the autumn.

CHSAA mem­ber schools had until 8 a.m. Mon­day to for­mal­ly declare for Sea­son A, which runs Octo­ber through Decem­ber, or for Sea­son C. The orga­ni­za­tion had not released a mas­ter list of fall foot­ball and spring foot­ball schools as of ear­ly Mon­day evening.

Sources told The Post that approx­i­mate­ly 31 out of 41 schools in 5A had opt­ed into Sea­son A, which can begin prac­tice on Thurs­day and will open its sea­son Oct. 8. The same sources said 37 out of 43 pro­grams in 4A were start­ing in the fall and at least 28 out of 40 in 3A.

CHSAA declined to con­firm which schools opt­ed to play the fall. A sched­ule for Sea­son A was expect­ed to be released this week, pos­si­bly as ear­ly as Tues­day morning.

Mullen elect­ed for Sea­son A almost as soon as the option became avail­able, coach Jere­my Ben­nett recalled with a chuck­le. He said the deci­sion was a “no brain­er” as a ges­ture to his upperclassmen.

“We’ve got some seniors that had lim­it­ed action last year but they’re col­lege foot­ball play­ers in terms of abil­i­ty, and I’ve got to get them on the film,” Ben­nett said Mon­day. “And real­is­ti­cal­ly, who’s to say the spring is going to be any dif­fer­ent from the fall?

“Our kids want­ed to play. And I want­ed that for them. And that’s why we made that decision.”

Den­ver Pub­lic Schools, mean­while, reaf­firmed its com­mit­ment to the spring, releas­ing a state­ment to The Post that read, in part:

“(The dis­trict) under­stands the desire and pas­sion around pro­vid­ing our stu­dent ath­letes with a foot­ball sea­son that will allow access to com­pe­ti­tion and recruit­ing for col­lege-lev­el schol­ar­ships. Cur­rent­ly in Den­ver, we are under guid­ance from our local health depart­ments that lim­it cohort size and adult inter­ac­tion with cohorts. This guid­ance makes it impos­si­ble to offer the same pro­gram­ming allowed in neigh­bor­ing com­mu­ni­ties and could lim­it par­tic­i­pa­tion oppor­tu­ni­ties for our students.”

Mullen has a rough idea of its sched­ule, save for the open­er, which Ben­nett said he under­stood would be assigned by CHSAA based on recent com­put­er rank­ings and oth­er his­tor­i­cal data.

Mean­while, 24 miles to the east, Vista Peak will turn its focus back to the spring. And toward a much small­er Sea­son C bracket.

“I assume (they’ll) be get­ting cre­ative,” Sul­li­van said of his sched­ule. “With what’s going on, I would hope so — I would be extreme­ly dis­ap­point­ed if (they’re) not. Gosh, we’ll play any­body. We’ll play 5A, 4A, what­ev­er. We would’ve been like that in the fall. We don’t care. We want to line up and play

“It’s tough for the kids. I feel hor­ri­ble for them. But we’ll make the best of it.”