Hey, Kadence!
Duelling: you won't be able to sign up for the tournament now, but if you do still want to duel, you can sign up for this term's
Duel Club. You won't be able to get a half level for it, but you will get an extra point to add into any future Duelling profiles.
Quidditch: you can sign up at any point during the term, so you can definitely go ahead and still do that.
Classes: it's totally fine to say that she's taking other classes in addition to the classes you're actually writing with her.
Deadlines: for Quid, no deadline; for Duelling, this term it was the 10th, but in some previous terms it's been the 15th -- it's not always the same, but usually within that window, so best to sign up as early as possible; for Spellbound, it's the 10th, but if you miss it, you can PM me/Lincoln Litchfield/Edmund Wolffe and ask to join at any point in the term; for HW, it depends on the Head Nurse -- again, they'll usually close around the 10th, but if you still want to join for this term, you can PM Rafael Blancamuerte about it.
(Generally, you can assume that sign-ups will always be open until at least the 10th.)
Hope this helps, let me know if you've got any other questions!