Sebastian Acevedo has achieved his impossible dream.

When his mom told a guidance counselor at Sebastian’s Catholic grade school in Queens that he was thinking about applying to the city’s specialized high schools, the counselor chuckled, saying, “It’s practically impossible.”

“She said it’s not gonna happen,” he told The Post. “I was basically dismissed.”

But the remark only fired up Sebastian, 13, to prove her wrong.

While finishing 7th grade at Saint Matthias School, Sebastian’s parents — his mom, Kathy, a Ecuadorian immigrant and clerical worker, his dad Eugene a supermarket manager — signed him up at Kweller Prep. The company gives classes for kids planning to take the SHSAT exam, the sole entry criteria for the specialized schools.

Sebastian “did terribly” on a Kweller placement test, he said. “I remember crying after seeing the results. But it made me try harder.”

He took the SHSAT classes after school, on Saturdays, and over the summer, sacrificing time for his amateur boxing. His parents also made sacrifices to scrape up the $5,000 cost.

It paid off. Sebastian scored 568 on the tough exam. Last week, the city Department of Education offered him a seat in September at Stuyvesant — which had the highest cutoff score of all eight specialized schools, 557.

He was one of 33 Hispanics among 895 kids admitted.

“I worked so hard for it. To have it finally happen was just amazing,” he said. “It was a dream come true.”

Sebastian also got offers from Bard HS Early College in Queens, a highly-selective public school, and four private schools on scholarships. He has chosen Stuyvesant.

His new goal — sparked by worrying about his grandma’s heart condition — is to become a cardiothoracic surgeon.

As for college, he said. “I’m interested in an Ivy League.”