which schools have most flexible courses requirements

It depends upon the school. Almost all schools give placement tests for math and foreign language and will be placed accordingly. Note that in some cases, the student may be repeating a topic at the BS. E.g… Just because they got an A+ in precalc at the old school does not mean they have mastered precalc.

Almost universally, the student will take grade-level history and English regardless of past coursework.