Because far more people die within the first year of life than they do in, say, the 20th year of life, the answer to your question about the percentage of people who die before they're 30 or 40 isn't really relevant to calculating the likelihood of all of your school peer group still being alive. (i.e.counting deaths from birth would count people who would have been in your school year group if they'd not died within the first few days or weeks of their life, whereas you're only interested in the number of people who might have died since leaving school).
These figures might be more useful:
The estimated number of 15 to 34 year old people in England and Wales last year was 14,386,900. The number of 15 to 34 year old people who died in England and Wales last year was 7025. That indicates that, every year, about one person in every 2000 out of that age group dies. Since it's a 20 year period, that means that about 20 people out of every 2000 will die throughout that period, i.e. 1%
So, statistically, you'd expect just one person from your school peer group of 100 people to have died since you left school.
Chris