It’s hard. My kids didn’t go to St Marks but I will echo what we were told “if you’re organized you can handle it”. I would be wary of this. Unless your kid is such a good athlete she can get into her chosen college with a B average due to recruiting. She will not be allowed to miss Saturday classes. She might be allowed to miss one or two, maybe three weekends in a semester. Getting to club practices on top of whatever school sport she’ll be playing on top of homework will be almost impossible.
You need to think really long and hard about what your priority is - St Marks or being a recruited athlete. As a day student she will need/want to spend as much time as possible on campus so that she is not left out of social groups. If she’s gone every sunday for club that’s going to be hard.
Specific questions to ask:
How many days per semester can my kid miss?
What do teachers do to help kids who miss a class? (Some teachers may help, some will put it all on the kid)
What is the afternoon activity commitment for each year of school?
Can my kid skip her afternoon activity commitment on the days she has club sports (I can’t imagine the answer will be yes but you need to ask)
Bottom line. Unless St Marks is below your kids academic level being a recruitable athlete at a boarding school is a grueling proposition. Boarding school is generally not set up to support the club sports lifestyle. Kids I know who have been successful had below average academic performance at school or were INSANE athletes to start with. Like had offers their freshman/sophomore year type kids. My kids would not have traded BS but they definitely let their athletics slide as a result.