One, your original statement makes no allowances for disabilities. To quote....

If you want a private business lisence you must pass a basic firearms safety and marksmanship course. If you want a home mortgage, you must pass a basic firearms safety and marksmanship course.
That's pretty much an absolutist statement. I don't assume, I'm not reading your mind. I'm just going by your statements.

Safe handling and markmanship would be part of the overall GPA from 1st to high school graduation.
Subjects other than basic education should have no effect on GPA whatsoever. If a student hates gym class, it should have ZERO effect on their GPA. Same with any other non-academic course.

As to the liability issue, liability insurance is a major cost factor in school sports. Many schools are now requiring parents to pay a participation fee for sports for that reason. The probability of a severe accident would go up exponentially if you put live fire ranges in schools. Even on military ranges, where controls are tight, and the people shooting are professionals, accidents happen, and accidents on ranges are rarely minor.

As an NCO I didn't trust others to train troops for me, and I'm damned well not going to allow my children to be taught firearms safety by someone else.