I was 15/16 in 1970 and as far back as I can remember, my mum always cooked an evening meal for us even though we had school meals until I was about 14.
We ate only seasonal fruit and veg as that was what you could get, we always had vegetables with our meals, we had biscuits and mum baked cakes sometimes and sweets but we didn't eat any of that to excess, just occasionally through the week.
We had puddings sometimes i.e. spotted dick, jam sponge, rice pudding etc. usually fresh fruit in the summer, strawberries being a favourite as we picked loads of them during the season as we were surrounded by lots of fields for picking.
Neither me or my brother had weight problems as we got into teenage years, we still did games/PE at school at least twice a week, we went by bus everywhere or walked or rode bikes.
We may not have had the choices we have today but despite the food maybe being stodgier, I don't think it had the additives or preservatives that we get now and as we were all so much more active on a daily basis, I think we burned off what we were eating.
I would say that out of my school year at that time, the number of overweight kids was around six or seven. Mainly girls, only two boys and by today's standards, they weren't really that big.