I do think people look at the past with rose tinted glasses.
There wasn't phone boxes "everywhere". They were in places with a lot of people. I had a 40 minute journey to school (10 minute walk, 25 minutes bus, 5 minute walk). The only phone box in that whole time was near the school.
And who to ring? Unless you had a stay at home parent... they would be at work, or on the way to work. If they worked in an office, they might have a phone you could ring.
We learnt to be self sufficient because we had to.
Mobile phones started to become more mainstream when I was in Secondary school. I can remember the one time my Dad phoned me on it, 10 minutes after school. September 11th, 2001, with the message to go straight home and to tell my friends to do the same (we lived in London).
Saying that, I do think mobiles do make life a lot easier. There is just no need for the vast majority to use them during the school day.