Looked it up cause fuck it. Japanese middle school covers ~12-15 years old and high school covers ~15-18. So, best case scenario is like a year between the two of them - worst case is she's 12 and he's 18... which isn't great to say the least
Edit: Chapter 3 confirms that she's 2nd year in middle school, and he's a first year in high school so it's nothing too crazy