Chitose Is in the Ramune Bottle, Vol. 3 (manga) - Hiromu - Bog

A battle fit for a king! Under Saku Chitose's strict tutelage, otaku loser Kenta Yamazaki is gradually learning to live the normie life. But when it turns out that a rival group of popular kids is coming after Kenta, it's once again up to "King Saku" to swoop in and save the day. Of course, resolving arguments and participating in some friendly competition are all in a day's work for His Majesty--but Kenta doesn't find it quite so easy!