"the boys 'practice' in the stadium" was not accepted. Practice is correct in UK English as far as I know, unless I am missing something it should be accepted?
"Practice" has been accepted for a while now (possibly because of your reports/comments). Thank you for helping us make this course better :)
Rapariga is for women and rapazes for young men. Where I live rapariga has a negative connotation.
Rapaz is common in Brasil too, for a teenager boy or a young man, but "rapariga" has an offensive meaning here and could be replaced for moça...
is "rapazes" a synonym of "meninos"? Or is "meninos" only used for young boys, and perhaps "rapazes" is more like an informal "homens"-- like "guys" in english?
the correct word is rapaz and is used only for young men. There is not this term for young women.
That's a great question, and it might linger on whether or not someone considers working out something akin to "training" (in the sense of getting better at a certain skill/sport vs. getting your body fit for whatever reason). Some people might use the term "treinar" when they go to the gym twice a week, but I don't think it's a correct descriptor of such activity, since "treinar" usually has a secondary purpose (to train yourself to do something else, or to gain the endurance to perform that other activity) guiding the activity.
Long story short: Working out is part of the training, but not the training itself (or not just).
"Fazer exercício" ou "Malhar" (in Brazil only) are better terms for "working out".