The mother of the Beacon High School student severely burned in a botched chemistry experiment testified that her son looked like “a monster” as he emerged in public for the very first time Tuesday.

A grim-faced Alonzo Yanes, wearing a baseball cap and blue Eddie Bauer windbreaker that all but covered his scars, and his parents arrived at Manhattan Supreme Court, where his $27 million civil trial against the Department of Education and teacher Anna Poole has been playing out.

But he wasn’t in the courtroom to hear tearful testimony by his mother, Yvonne Yanes, about how she dealt with the aftermath of the 2014 accident.

Her daughter Alana, who was 7 at the time, was begging to see her big brother, Yvonne told jurors.

“‘Alonzo does not look the same anymore. He will probably look very scary to you. He will probably look like a monster, like Frankenstein,'” the mom recalled warning her daughter, as her voice broke. “‘He has been patched up a lot. He has staples in him. So he is going to look very different. I just want to let you know, it just might be a little scary.'”

Yvonne said it took a moment for Alana to warm up to Alonzo’s new appearance.

“Alonzo came into the room and I could see Alana stiffen up and Alonzo lunged to go hug her and she kind of stayed away at a distance from him until he started talking with her,” she said. “You could see visibly Alana’s body kind of relax as she responded to her brother’s voice.”

The mom burst into tears as she recalled one of Alonzo’s friends coming to visit for the first time about two months after the failed experiment.

“Some accepted him and some were clearly taken aback by how he looked and his burned appearance,” said Yvonne. “But they all tried to be very kind. There was no embracing because he is still sensitive to touch.”

She added, “I didn’t stay around for very long [that day] because it was so painful to watch.”

Alonzo was 16 when the fireball erupted in the Upper West Side classroom during a procedure called the “Rainbow Experiment.” Poole is accused of causing the fire by pouring flammable liquid from a large, fume-filled jug onto a lit crucible.

The now-21-year-old suffered third-degree burns on 30 percent of his body, including to his head, face, neck, torso and hands, according to court papers.

Yvonne told jurors that she tried to dissuade her son from looking at himself in a mirror — fearful of how he might react.

“I tried to prolong that for as long as I could,” she said, “because I don’t want him to see what I was seeing.”

“The face, the way that he looked … I didn’t think he could handle it. He was 16, he was a kid, a teenager,” the mom explained. “He had just come from a point where he was trying to work on his physique. Now he’s 20 steps behind from where he was starting up from. Now he’s got a longer way to go now with the damage that was done to him.”

Alonzo is expected to take the stand as early as Tuesday afternoon.