For reference, there is precedent for a bigger comeback than this.
Barcelona vs. Paris Saint-Germain, 2017 round of 16 of the champions league
Barcelona was down 0-4 after the first leg, and ended the game 6-5.
With that said, Barcelona was, generally speaking, already widely regarded as the best club in the world. Meanwhile, Seiseki is the clear underdog in this match with zero players from the high school national team. A comeback is definitely painfully unrealistic and I do hope the author picks the realistic route and has Seiseki barely lose, after a "near-comeback". Tsukamoto can have another go at it when he's older and better.