First, you mention senior trip, so I just am going to mention that you and your friends all need to be over 18. If one of you is still a minor at sailing, they need someone over 21 in the stateroom.
On price: it is very likely that there will be restricted rates. You can get an idea with what they might be by looking at this year’s Transatlantic. They have restricted rates for all categories. Most likely these prices will come out after pay in full date for the cruise, so you would have to wait and gamble a bit. But I think in recent years all EBTA ended up with guaranteed rates at some point.
Flights back: first, if you are young a 9 hour flight should be easy to deal with. Don’t let that deter you and your friends. Also, the flight to the USA is usually easier as you just spend a long day on the plane. On price: getting back from Barcelona after deboarding is difficult anyway as most flights to the US leave in the morning. I think DCL recommends not a flight before noon and I would follow that advice for a TA cruise. The ship needs to be cleared into Europe when arriving and you have to go through passport control and customs. So, if you want to get back immediately you won’t have a lot of choice.
There are a few cheaper airlines: Norse (however, they might not be financially stable, so only buy a ticket fairly short notice with them, not a year out), Iceland Air and TAP often are lower priced than standard carriers. But only TAP would offer flights from Barcelona. They actually do have a program where it’s easy to book a flight with a layover in Lisbon. But if you want a cheap flight, you might need to get yourself to London or Paris first. It might still be cheaper to fly there and then onwards with a cheaper carrier. Of course this adds accommodation costs, too. Be careful when booking European cheap carriers: most of the time it makes sense to book a fare that includes luggage as the fees for addding luggage later are higher than the price difference for the fares. And adding luggage at the airport is prohibitively expensive. Also some only allow a personal item, if you bring a roller suitcase into the cabin, they charge for that, too.
The cheapest way to fly one way across the Atlantic is by miles. A lot of airlines have credit cards that give you a sign up bonus of a nice amount of miles (which should cover the one way ticket) if you spend a certain amount in the first few months. If you are as young as you seem to be, this might be something for your parents yo look into. Usually the airline miles programs do allow to book tickets for other people, especially family. There is a whole thread about credit cards on the Budget Board on here which has heaps of info, also about what not to do!
. Most students wouldn't have the established credit to qualify for these offers, anyway.
