I am sure that NCL will say it is possible to book large groups of any age never mind groups of teenagers as busines is busines! Also although NCL market their cruises as free style dining which means that you can choose at what time, and at which dining room venue that takes your fancy. You can also firm a time prior as opposed to standing in line if you wish. I am assuming that the Spirit policiy on dining options are the same as on board the Jade & the Jewel, which we have been on and that the Windows Dining Room is not a cover charge restaurant.
Oh! I have just been reminded not to book a cruise on the Spirit in the months of this coming Feb/Jan for fear of being invaded with a mass of teenagers!!!! Only Joking!
February is also a very popular time to take children and teenages on holiday due to the half-term holiday. My children travelled on NCL last year and had a super time as they were very well catered for with plenty of activities both inside and outside the kids club. Given the vast numbers of under 18's on board it looked like other parents were also enjoying this opportunity for a family break.