Just the fact that in all studies involving kids and computers you have the Hawthorne effect makes it difficult to separate the positive effects of giving kids computers from the positive effects of giving kids more attention. Various studies show that kids improve their learning using other techniques, without computers. Some schools have demonstrated high performance in learning with a de-emphasis on computers. We really don't know whether computers improve learning or not. I wouldn't say we'll never know, but the factors going into learning are so complex and interrelated that it's hard to determine.
Teacher training is essential, but currently undervalued and under-budgeted.
Another important feature that is undervalued is technical support for teachers and kids. Most schools can't afford technical support. What really makes the difference are high-quality teachers, small class size, and parental engagement. If you want to have kids learn computers, then you need good teachers and a room full of computers. But if you're simply trying to get kids to learn traditional subjects, there's no indication that computers really make a big difference.