Hi Teryndactyl,
I?m sure you found something by now but for anyone else in this situation I would recommend Inhispania. The school is in central Madrid and, although, it isn?t cheap (though this school is one of the better value establishments) it is definitely worth the money. It is hard to distinguish between them all because there are so many good schools in Madrid. I did a course at Inhispania myself ? both the Intensive 20 (20 classes Mon-Fri 10am-1pm) and then switched to 4 hours a week in the evenings (7-9pm Mon+Weds/Tues+Thurs). I found the teachers to be really personable and interested in you and they really cared about your progress. A friend at a different school hated her classes because the teacher would shout at them!
The central location is ideal - less than a minute?s walk from Sol metro ? and furthermore I?ve found it to be one of the more reasonably priced.
The type of students you will find is a mixture from aupairs and erasmus students to grown adults so don?t worry about fitting in; plus they offer extra curricular activities so you can meet people there as well as practice you Spanish!
All the practical info is on http://www.inhispania.com but there you have my personal recommendation!