But personally, I would recommend intense and sustained cardiovascular exercise.
But it did change my life at that critical I-don't-know-how-I'll-make-it-through-university moment. Especially when everyone around me offered stellar advice like "try harder" and "don't buy in to the Big Pharma ADD/ADHD-conspiracy".
It turns out that I'm also reluctant to just offer "exercise more!" or "work on yourself -- it's your behavior!". It's frustrating and anxiety-inducing enough to struggle with motivation. If the only solution is to put pressure on such people by suggesting it's all in their head, then what do we do if that pressure doesn't actually fix anything? Do we count down to the catastrophe so that we can chime in one last time with "See? Look what you did."?
Exercising and taking a sabbatical are good advice, but sabbaticals are out of the question for most people (especially a kid in highschool), and exercise is only that -- good advice.
At some point we have to admit that the results of no-improvement will out-trouble our concerns/stigma over last resorts like medication.
What OP was talking about was not psychostimulation, but concentration enhancer, something that would just reduce effects of "mind wandering" and let him do tasks that he doesn't want to otherwise.
Aniracetam and oxi do greatly reduce effects of "mind wandering", large amounts of grains and fruits give your brain very much glucose, i.e. energy to use to, coffee increases your metabolism and blood flow, and gives a slight stimulation, so you can force yourself to do task more easily.
Don't overuse coffee, though, it's effects can last from 1 to 3 days (or even more).
Being arrested for possession would have some negative consequences for OP. Accepting a caution would be worse.
So, if OP does want to try this be careful about the legal status and if the police do call be polite but say, firmly and clearly, that you'll answer their questions after you've spoken to a lawyer, and refuse to say anything else.