OP, if you’ve taken unofficial visits to high academic schools and grades haven’t come up then you’re at a very preliminary stage of recruitment. No coach is going to waste his time recruiting someone that won’t pass muster with admissions, hence, the first thing that any coach who is interested in you as an athlete at a high academic will ask is what your test and grade scores are.
There is no hard and fast rule, and it depends on the sport, but in general, the better you are as an athlete, the more leeway you are given on academics. In other words, if you are a nationally ranked offensive lineman who is 6'6", 290lbs., then a B- would probably be no biggie. If you are a slightly above average 5'10" RHP throwing 84 MPH, then it won't matter, as even if you if you get straight A's you more than likely aren't a good enough athlete.
Bottom line for you, though, is that if you have a 3.7 GPA with a rigorous course load(i.e. at least 5-6 AP courses), a 32-33ACT, and you are an outstanding athlete compared to your peers around the country, then you have a chance to be accepted into an IVY. It really matters what sport we're talking about, also.