RIVERDALE, Ga. - Colin Kaepernick’s saga took another surreal turn last Saturday — a last-minute audible to nix an NFL-arranged workout and a quick dash 60 miles to the other side of Metro Atlanta, where the exiled quarterback staged his own impromptu passing display on a high school field in dwindling light as hundreds of fans cheered him on from behind a chain-link fence.

Mr. Kaepernick threw passes for about 40 minutes at Charles Drew High School and spent nearly that long signing autographs for a crowd that steadily grew as word spread that a quarterback who led the San Francisco 49ers to the Super Bowl and sparked a wave of protests and divisive debate by kneeling during the national anthem was in the neighborhood.

Mr. Kaepernick declared again that he’s ready to play in the NFL — if someone will just give him a chance.

“I’ve been ready for three years,” he said. “I’ve been denied for three years. We all know why. I came out here today and showed it in front of everybody. We have nothing to hide. We’re waiting for the 32 owners, the 32 teams, (Commissioner) Roger Goodell, all of them to stop running, stop running from the truth, stop running from the people.”

In a move no one saw coming, the league that Mr. Kaepernick claims blackballed him called Nov. 12 with a take-it-or-leave-it offer to hold an extraordinary workout at the Atlanta Falcons’ training complex in the sprawling suburbs north of the city.

All 32 teams were invited. Former NFL head coaches Hue Jackson and Joe Philbin were set to run the drills. The league said 25 teams were sending scouts and representatives, many of whom had already arrived at the Falcons’ indoor training facility in Flowery Branch when word came that Mr. Kaepernick wasn’t going to take part.

Instead, Mr. Kaepernick decided to hold a workout open to the media— unlike the session at the Falcons’ facility— at a high school stadium just south of Hartsfield-Jackson Atlanta International Airport, 60 miles away.

With barely more than an hour to spare, the media that had been herded into a gated-off area in the parking lot and told that was as close as they would get to Mr. Kaepernick’s workout hustled to their vehicles to set off for a high school most had never heard of.

But only eight of the original 25 team representatives followed along to the new location, including Philadelphia Eagles vice president of football operations Andrew Berry. It appeared the New York Jets, Kansas City and Washington also had scouts in attendance.

They stood along the sideline, jotting into their notepads as Mr. Kaepernick tossed passes to four free-agent receivers.

“Our biggest thing with everything today was to make sure we had transparency in what went on,” Mr. Kaepernick said. “We weren’t getting that elsewhere, so we came out here.”

Mr. Kaepernick has clearly kept himself in good shape during his near three-year layoff. His passes had plenty of zip on them, though he was a bit off target with a few of his deep throws. It was not the sort of session that would likely sway a team one way or the other.

That didn’t appear to be the point.

Mr. Kaepernick has insisted all along that everyone knows he is good enough to play in the NFL. He claims this is all about his decision to kneel as a way of protesting police brutality and racial injustice, which led to a contentious national debate that stretched all the way to the White House. President Trump called on NFL owners to fire any player who failed to stand during the national anthem.

“We’re ready to go anywhere,” said Mr. Kaepernick, who at age 32 should be in the prime of his career. “Ready to talk to any team, interview with any team at any time. I’ve been ready, I’m staying ready, I will continue to be ready.”

Mr. Kaepernick did not take questions from at least 50 media members who scrambled to get to Riverdale to cover his workout.

The quarterback’s representatives announced the change less than an hour before the start of the originally scheduled workout, saying the NFL refused their request that journalists be allowed to watch and videotape the session in Flowery Branch.

When he finally got on the field, Mr. Kaepernick threw passes to receivers Bruce Ellington, Brice Butler, Jordan Veasy and Ari Werts.

Former 49ers teammate Eric Reid, who joined Mr. Kaepernick in his kneeling protest and a collusion lawsuit against the league, watched the session from a bench on the sideline.

Mr. Reid, who is now a safety for the Carolina Panthers, left the workout about halfway through to get back to Charlotte, N.C. His team, in an interesting twist, will host a game against the Falcons on Sunday, Dec. 8.

“I think there could be a positive outcome,” Mr. Veasy said. “At the end of the day, any progress is good progress.”

But Jeff Nalley, Mr. Kaepernick’s agent, didn’t sound as hopeful. His client hasn’t played in the NFL since the 2016 season.

“If teams want to see him, they will ask to work him out,” he said. “No team asked for this workout.”

Mr. Nalley said he feared all along that there was “an ulterior motive” behind the NFL’s offer to stage an unprecedented, combine-style workout for one player, especially because Mr. Kaepernick was given only two hours to accept or reject it. The league turned down his requests to move the workout to a later date or to hold it on a Tuesday, when more coaches and general managers likely would be able to attend because that is an off day for most teams.

In a statement, the NFL said it was “disappointed that Colin did not appear for his workout.” The league referenced recent negotiations with Mr. Kaepernick’s representatives over the workout and cited issues that included media availability and a liability waiver.

“Colin’s decision has no effect on his status in the league. He remains an unrestricted free agent eligible to sign with any club,” the NFL said.

The league in February settled a collusion grievance filed by Mr. Kaepernick and Mr. Reid.

Mr. Nalley acknowledged that he had already made arrangements for a different site if talks with the league broke down.

“You’ve always got to have a backup plan,” he said.

Mr. Kaepernick’s representatives said the NFL “demanded” as a precondition for this workout that he sign an “unusual liability waiver.” Mr. Kaepernick also wanted his own film crew to attend and videotape the session, which the NFL rejected.

Mr. Kaepernick worked out under the lights as the sun set behind the trees at one end of the high school stadium, an American flag flapping in a gentle breeze.

At the other end of the field, Mr. Kaepernick was cheered on by a fan contingent that kept growing throughout the session. A police unit arrived to control the crowd. A barbecue truck set up in the parking lot to provide an impromptu dinner option.

When he was done, Mr. Kaepernick worked the entire length of the fence, signing hats, shirts and any other item that was tossed over the fence for his autograph.

One of the fans held up a sign. “I’m With Kap.”

AP Pro Football Writer Rob Maaddi contributed to this report.