I have been on mexillitiine for quite a while, coming up on 11 years or so(I'm 19 now). As a child I didn't notice anything major except for relief, but now that I'm older I really see the effects it has. I do fatigue quicker than the average person, but with proper exercise and diet it can be improved greatly. As for school, I found it didn't have any effect on my studies, unless I missed doses cause then I had a hard time functioning. I am currently in university going for a engineering degree. The one thing I do want to give caution about from experience, I am completely dependent on this drug for everything I do and would advise to try and learn to live with myotonia instead of using medications.
There are many side affects, luckily I didn't experience much of them which made this one a great choice. I did a quick search on the one I use, mexiletine HCL, http://www.rxlist.com/mexitil-drug/side-effects-interactions.htm. This does have a lot of info that can give you some answers that you may be looking for. Thanks for reading.