I keep getting the feeling that the reunion happened before the the girl taking her class hostage. Like they getting together was a set up for her to get used to blowing people up or something (?)
Early on it said "meanwhile" when it switched from the reunion to the girl in her class. "Meanwhile"--unless it was a bad translation--means it's happening at the same time.
*people explaining the plot*
Me: https://i.kym-cdn.com/photos/images/original/001/242/303/06a.png
I didn't think it was TOO confusing. :)