Can't you put your current ones in the washing machine, then any stains should go or what i've done before is wreak them playing sport or purposely chop them up but make it look like the heel or soul has just busted, then your Mum will have no choice but to get you new ones
A stain is something you can change, especially if that's the only thing you don't like about your sneakers. Ever thought of cleaning them? Washing machine, sponge and soap, etc.?
They're kids out there who don't own any shoes and you're sad because you can't get a new pair? Be thankful for what you have.
show her your ruin shoes and tell you can't wear them and i know she well understand after all she is your mother
start using your own money
Ruin your shoes so that mom buy a new one. I used to do that when I was a kid
