I actually wrote a post about the benefits of ed tech in education for grad school several years ago, which addresses this issue: http://blog.wsd.net/jreeve/the-case-for-ed-tech/
The TL;DR is: Yes, they undeniably improve academic performance, and studies have shown this repeatedly. However, keep in mind the studies demonstrating this are done with teachers who actually knew how to use the technology, were trained to do so, and had specific instructional goals in mind for using the tech to benefit their students. If schools/districts just throw this technology at teachers without proper training and academic objectives, it will do nothing. Computers and tablets are useful in the hands of a good teacher, but they are tools. They cannot turn bad instructional practices into good instructional practices.