As someone who was diagnosed with ADD at age 10 and eventually discovered my own ways of beating it, I'll share a few ideas. But note that what works for me, or someone else, may not work for you and you'll have to slough through things to find your own fix.
Band-aids:
- Caffeine (tea is best since it's like an XR version of coffee)
- Testing for any vitamin/mineral deficiencies that may be causing tiredness that's displaying as an inability to focus, like low iron or b-vitamins (just examples, go see your doctor)
More longer-term solutions:
- Read "Getting Things Done" and re-read it until you internalize it
- Use an app like OmniFocus to organize to-dos and give yourself a sense of achievement/little adrenaline rush when you complete something (especially small things, like make bed, read 25 pages of book, one homework assignment, etc)
- Change your study habits: turn off your phone and put it in the drawer, disconnect from the Internet if possible, relocate to a different location where there are fewer distractions (coffee shop, library, etc), give yourself small blocks of time to study (after all, work expands to fit the time allocated)
Longest-term solution:
- Gain enough life experience, self-awareness, and fear of failing out of school that you eventually figure out what works for you.
Hope that helps!