Hi Lari,
There aren´t that many Spanish schools in El Salvador, I am sorry to say. I found one for my friend that had private classes in San Salvador and also Santa Ana but it was quite expensive, like aroun $20 an hour.
You are much better off going to Antigua Guatemala where you will be paying a lot less for very good classes. There are about 70 schools there and they provide home stays and I think it costs about $120 to $150 per week of private classes.