you are actually doing it all wrong, my man.
You need to study sober and take the test high.
I did this and got 100% on my math test in high school.
How this works is that you need to preserve your short term memory, so you study sober. Once the test comes, take some of that weed and this will help enhance your long term memory. your long term is what you'll be using for the day of the test.
Hope this helps