If you go to most universities, psychology is a part of the faculty of Humanities, not science. There's a reason for that.
To call a discipline a science, it needs to use the scientific method, not just sometimes, but always. It needs testable hypotheses. Psychology sometimes has this, but often it does not. There is certainly reason for non-science fields to use scientific methods at times, but that doesn't necessarily mean those fields should be called sciences.
An illustrative example is the difference between "medicine" and "medical science". Your doctor has studied medicine. He or she, in addition to studying some medical science, has learned interview techniques, psychology, and various other aspects of a craft that are, in no way, scientific. If you talk to a doctor in a non-medical setting (many are specifically trained not to reveal ignorance to patients in order to maintain patient confidence), you'll find they're remarkably ignorant about the how's or why's of the human body, except when it comes to something they've been trained to spot and fix. A huge portion of their training centers on knowing when to do nothing at all, because medical intervention almost always carries it's own risks. In his or her daily job, your doctor does not employ the scientific method. At least, you should probably hope you are not being experimented on by your family physician! Some doctors do research in the field of "medical science", but this really is an entirely different job from being a family physician, surgeon, etc.. Medicine is a highly skilled craft that sometimes uses science, but it is not itself a science.