If you're already on medications, I would check with your doctor to make sure there won't be any interactions. I think it unlikely you would develop serotonin syndrome unless you were on antidepressants or other medications that affect serotonin (serotonin receptors are present in the gut as well). As with any antidepressant (St John's wort might be natural, but it's still a medicament), your mileage may vary, and it can take up to eight weeks for it to take effect.

I was on St John's wort for about eight weeks; didn't do anything for me. Could be because I took the wrong amount, or because I didn't get a properly standardized brand, or because my body just didn't care for the plant. It has worked for others. If you do start taking it, make sure that you've chosen a good brand that has the standard percentage of hypericin and hyperforin content (the Internet is your friend here).

I hope everything works out before school starts. School's already a handful without depression tagging along. And if you haven't, you might want to start seeing a school counsellor. They can be really helpful. And if the St John's wort doesn't work, maybe talk to your doctor about medications, or, if you can afford it, a psychiatrist.

Good luck!