Is it necessary to take AP Biology before the senior year, if the student wants to apply for pre-med or direct BS/MD programs.
No. You get no brownie points for taking it before senior year. For that matter, it is not necessary to take AP Bio at all.
But its nice to have the subject test in Bio done before applying, isn’t it?
It’s nice to have strong Subject Test scores if the colleges asks for Subject Tests. There is no college that specifically asks for Bio. And there is no pre-med major to be admitted to. So unless a college asks for specific Subject Tests, the student should focus on those in which s/he will score best.
That said, a student planning on pre-med will be well served having a strong HS science background with bio/chem/physics,and at least one at an advanced level. But s/he will also need a quality foundation in other core subjects as well.