It really depends on the school. Some require jacket and tie, others a collard shirt. Sweaters but no sweatshirts, that kind of stuff. You really have to look into each school’s policy. For the majority of schools I would say the dress code these days is “look clean” - no PJs, sweats (though generally things with school branding are allowed), ripped or too short. Our school used to restrict big logos/graphics, put a pocket T was ok.
As far is the blue jeans thing is, “denim” is allowed - it’s kind of like wearing chinos/khakis. My kids generally opted for colored jeans as opposed to anything more formal.