How many tacher training institutions do you need?

There seems something odd about this quote from the Claudio Sanchez piece I talked about in my last post.

There are 1,300 schools of education

Population of USA = 300M
School age population = 10%, say = 30M
Teacher : student ratio of 1:10 = 3M teachers
(I am guessing this is the ideal, while the reality is 1:20)
Avg lifetime of a teacher = 30 years.
Number of new teachers needed per year = 3M / 30 = 100,000

If the number of teacher training institutions = 1,300,

then number of teachers per institution per year = 100,000 / 1,300 = 70 . Huh?

Even if you are losing 50% of your teachers on the way, that is only 140 teachers per institution per year. That seems awfully low. And remember, I am using the 1:10 ratio for teacher:student.

What is wrong with my math?