Intervention to prevent depression in adolescents

Lets say, I’m interesting in an intervention to prevent depression in adolescents.  I give 1 school a CBT intervention (intervention group) to prevent depression and another school  psycho-education (control group). My IV has two levels (treatment/control) and my DV is scores on a reliable and valid measure of depression. I do some fancy statistics and conclude that at the 3 month follow, average scores on the depression scale were significantly lower in the intervention group than the control (p<0.001).

Given what we know about the limitations of quasi-experimental designs can I conclude that the intervention is effective? Should I tell all counsellors to use my fancy intervention? What can I conclude? 

Can you include references so I may read them?  Thanks so much