Here's how I rank cruise atmospheres from what you've asked about. This goes from best to least.
1. Spring Break- My spring break cruises have always been the most high-energy cruises out of all. Theres always a high count of teens and theyre always wild and out there. It seems like people always go on spring break with the intention of leaving all cares back home because come on- it's "Spring Break".
2. Summer Cruises- These are right behind Spring Break and are usually a lot of fun. It depends on when you go. My recommendation would be to go mid-summer. In July most schools are out whereas early June and Late August many schools have started again and you will have a smaller teen group. Summer cruises are a bit more laid back because theres no pressing school issues when you get home thus LESS STRESS. And yea, some of my best hookups have occured during the summer months.
3. Winter Cruises are the least profitable because there are usually lower teen counts and lots of the teens onboard stay with their families as a result of the pending holiday season. Also, the ships are less active as the cooler weather discourages some activities. Even in the south caribbean, on the Mariner this Dec. it was cool overall and there was little activity by the pools and stuff. However cooler weather seems to spark emotions and such and there is some serious romance on some winter cruises...