I'm just an ordinary middle school student with no particular aptitude for understanding.
Literal
Particularly understanding-ability [subject-が] exist [it's not that-訳でもない] ordinary middle-school-student [copula-です].
理解力 'comprehension ability.' わけでもない 'it's not particularly that' — a hedged self-deprecating denial. 普通の中学生 'an ordinary middle school student.' Self-effacing introduction typical of forum posts asking for help.