There is a limited amount of research on this subject. Below are 3 good examples:
Clark, W and Luckin, R. (2013), 'What the research says - iPads in the Classroom', London Knowledge Lab.
An excellent overview of all the research so far on this subject. The summary really articulates the nature of iPad and what they offer. If you read one paper, this is the one to read.
Burden, K, Hopkins, P, Male, T, Martin, S and Trala, C. (2012), 'iPad Scotland Evaluation', University of Hull.
A very long paper providing evidence of practice across 8 schools in Scotland. This is the largest scale study currently available.
An in depth study of iPad implementation and use at a secondary school in Kent, England.