Call it the summer of CSS.

Twenty teen girls from underserved communities in New York City will spend eight weeks this summer taking intensive technology courses, on topics from robotics to app development, as part of a new initiative called Girls Who Code that aims to increase the number of women in technical fields.

Efforts to close the gender gap in tech often focus on revamping schools' curricula to emphasize STEM (science, technology, engineering and mathematics) education. But former New York City deputy public advocate Reshma Saujani, founder of Girls Who Code, argues that schools are still falling short, graduating students who have little access to computers, lack basic technical skills, and see science and math as a pathway to a career in medicine and little else.

"When we talk about STEM, we talk about science, math and engineering, but we don't focus on the 'T' -- the technology part," Saujani said. "We fundamentally believe computer science should be mandatory in every high school, and this is the beginning of that movement."

During their two months with Girls Who Code, the 20 teens, who range from 13 to 17 years old, will learn how to build websites and mobile apps; tour tech companies such as Facebook, Paperless Post, Twitter and Google; and hear lectures from Twitter CEO Dick Cosotolo and Gilt Groupe founder Alexis Maybank, among others. Recruiters and engineers will also speak with the young women about skills needed to succeed in a male-dominated field, and the program participants will be able to pitch business plans to engineers on a dedicated "if girls ruled the Internet" day.

Saujani defended the decision to focus on females -- rather than opening up the program to their male counterparts -- arguing that increasing the number of women in tech should be a priority because it will help kickstart innovation by bringing fresh brainpower into technical fields.

"From the beginning, we wanted to create this sisterhood," Saujani said. "You have 56 percent of the labor force that you're not utilizing, and so you're missing out on innovation ... We need everyone operating at their top potential, which means we can't have an entire gender not opting into this field."