My Passive Skill Explosive Growth

The world is large and modern busling with adventures ready to face the fights in gates that appear, where inside monsters lurk from another world. With the appearance of these monsters came hunters ranking from F to S Class. If these gates aren't taken care in 7 days from when they appear they open causing a dungeon break and sendingnthe monsters into the real world full of civilians and life. The job of a hunter is lucrative since the essence stones that monsters drop depending on the rank of the dungeon will let you spend them for large sums, and mana stones can be sold for a lesser price but still valuable since they create clean energy. The essence stones sold are used to make valuable and powerful weapons and can be sold for even more. Our main Character is striving to be a great hunter but is lackluster in any skill and even with a sharp eye and good reaction time and instincts he is only rank F and too poor to afford equipment. Upping your rank in this world takes time and effort some hunters even taking years just to rank up once. Right now our young protagonist is a freshman at a high school for training hunters. And he is the lowest rank of hunters ever not even just the school a simple mana meter shows smaller scales what your power level is on aposed to an evaluation which determines your rank overall. And F ranks usually get 40 at least. He only got 5. Right now were in class getting bullied by the schools 10th place. He's a powerful tank and even has the rare skill for a tank called taunt. He is able to over power our MC a billion to one. Not related to the current situation our MC gets a cheat skill four week later at a random time during the day and its not a skill more of a passive skill where his leveling grows is 50 times the average hunter called "explosive growth" also that happens four week later right now hes still in class. Also time is different. 1 hour outside is 1 day in a gate.