"Surferos Surf School" is managed by Alex, the owner of Esencia Nativa in El Zonte. He and his instructors have a lot of experience and an official ISA-certification. They offer custom made surf lessons for every level. As a beginner, you will start with two theory lessons.
Puro Surf is probably the top of the surf schools in El Salvador, providing individual surf lessons. Their facilities are unique in El Salvador: a skatepark, a gym, a pool for practicing the paddle technique, and a lot more. The prices are probably not for the budget of a backpacker, but if you intend to be the next Kelly Slater: go for it!
Phone: (503) 6148-7782