It depends on how old you are and if you are doing any after school activities. If you are involved in something like a sport or club where you would need to be picked up, it would help to let them know if you got out early or if a friend;s parent has offered you a ride home. Or if they couldn't pick you for some reason but made arrangements for someone else to get you.
Your parents don't have a phone? When I was a kid in the 1950s my parents were one of the last around to get a phone put in. Sorry about your situation, maybe you could take up a collection at school and help your folks get a phone.
if you walk home it is probably a good idea to get a phone so just tell them a whole lot of bad stuff that could happen to you without a phone and then tell them how the phone will help. also you might want to consider making a deal with them like ill pay half you pay half sort of thing