Our own (slightly biased) recommendation!
In our annex, the Casa San Andres, we rent a couple of rooms (by day, week or month). The available rooms are very big and overlook the enclosed courtyard (see photo above). There is a fully equipped shared kitchen and shared bathroom and the house is 4 minutes walk from central park. There is also a studio apartment (with private bathroom and kitchen). The rooms can be rented through airbnb but we offer special prices for students so email posadaxela@gmail.com for info about the rooms.
Buganvilia Room (smallish room but very light, most economical)
Cala Room (huge room, normally rented long term, 2 weeks plus)
Orquidea Room (another huge room, normally reserved for short term stays)
Other recommendations
Short term
Black Cat Hostel is less than a block from the school. Dorm rooms start at Q60 a night including a delicious breakfast. www.blackcathostels.net
Casa Seibel is a beautiful colonial house with dorm rooms and private rooms starting from Q50 a night. www.casaseibel.com
Long term
For students who are here more long term (ie more than a month) we also recommend rooms at the local non-profit Entremundos. Also an old building with furnished bedrooms and all services including wifi and use of computer room. For more information go to www.entremundos.org or email: volunteering@entremundos.org