As Eric drinks the water, his strength slowly regains back to what it was when he was 15 originally. He also slowly grows back to his normal age, only to be embarassed even more since he was still wearing is ten-year-old boy clothes, so they were relly tight on him.
It took a really long time with the negotiatings between the high school and elementary school principals, but Eric was finally able to resume his high school career. To prevent anyone from knowing, he tells everyone that he was on a really long vacation.