I used to have the same issue when I was 15, but my mum would make me walk to school and back with my brother even though he was younger than me and the walk was ten minutes or less.
Does your mum walk you home after school too? If so, tell her you have an after school revision session or a club so you'll be back an hour later so she won't be waiting for you at the end of the school day. Then just walk home yourself when school normally finishes. Obviously lying to your parents is bad and all but your mum needs to learn to back off and this way you'll at least prove to her you can walk home and be perfectly fine. Its what I did and it worked, hopefully it'll work for you too.
Good luck.