I've never noticed any sort of concentration help with tea, but then again, I've been drinking the stuff since I was on the bottle. Might make a difference there. Also, I don't put sugar in it.
As far as increasing your concentration, I've personally found that a regular sized can of Full Throttle and a large beef jerky link is like manna from heaven. Made it through many a college course on that setup alone.
The truth is, you need to find what truly works for you, not what works for other people. Whatever it is about your body chemistry that is uniquely throwing off your ability to concentrate is what needs to be fixed, and it could be stress, could be sugar imbalances, could be to much or not enough caffeine... ect ect ect.
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Every time you think you've made it idiot proof, someone comes along and invents a better idiot.
?the end of our exploring, will be to arrive where we started, and know the place for the first time. - T.S. Eliot