There were some kids their parents tried to push them into our gifted class (as a tutoring center, we had regular classes too), and they got borderline scores,

There were some kids their parents tried to push them into our gifted class (as a tutoring center, we had regular classes too), and they got borderline scores, so we let them in. They could not follow after a while. I would say they did alright in later schooling but not to the same level of academic success as those in gifted classes. There are few Academic Olympiad gold medalists from the city. In fact, I only know of these two who were my own students so far and another bronze medalist many years previously. (From his later scholarship--he is now a math professor--I think he would have earned a gold medal had he have gifted education back then.) That said, it was not a controlled experiment so you can think of it as a case study but not gold-standard randomized controlled trial. The test was a set of challenge-style math problems for primary kids. Solving them would require more creativity and facility with thought than regular math problems.