Amber,
Seville is one of the most beautiful places in Spain, it is a great city, and not so big that you would lose the overview. You can basically do everything walking which is nice.
You can stroll through empty, narrow cobblestone streets at night and feel as if you were in another world. The school arranges for accomodation and it typically in old Sevillean houses- mine had a little courtyard and mosaics on the floor and was a 5 minutes walk from the school. I lived with an elderly lady from Seville which was also quite an experience.
The climate is much milder in winter and spring than in many other parts of Spain (including Madrid), but in summer it is supposed to be terribly too hot.
The food is wonderful-very close to the school there are great places to have some small dishes. And then the pastry places...
As for the Spanish, you might find the accent of people from Seville harder to understand than the one from Madrid or Northern Spain. However, the teachers do not speak strong Andalusian, but are aware of the fact that they have to teach standard Spanish (although they are proud of their roots in Andalusia).
In language schools, I think you usually meet a lot of quite young folks- when I was at CLIC, there was a predominance of people in their early Twenties, particularly from Sweden, who stayed their for an entire year. However, since the school is big enough to have a whole range of people, after a few days I had my group of people in their late twenties and thirties and we had a great time. The school organizes quite a few social activities, so it is easy to get to know people.
The teachers were very, very nice. I hardly ever saw so many nice teachers at one place. Friendly and good instructors, and they even took us out at night.