Don't joke about the Isekai shit. Dude spent like two volumes building up a fun stable of characters and then just fucking killed them all and started a different story. It absolutely could happen.
Also if I was suddenly transported back to my high school days math would be the scariest thing for me too. I did very well in math and took it up to calculus. But I uh... don't use it all that much anymore, so I'd be dropped straight into things that I no longer remembered how to do.