What to do if i have to repeat pe 'cause i'm "underweight"?

PE and low weight: This does not sound reasonable: repeat PE because you are underweight? Good exercise AND healthy eating are essential for good weight ( some blame can be placed on the genes you may have inherited if you have a small build). Please verify what is being required of you at PE

Get a lawyer: If this has been mandated by a beancounter because of some "ideal weight" chart, your physician can write a letter explaining basic biology. I'm a physician caring for athletes and know that many elite track-and-field athletes are "underweight". It's your build, your genetics, and your sports preference. It's not a measure of actual health or even fitness. Best wishes.